We called my dad’s mom Granny or Nana.
We called my mom’s mom Mammo or Big Mary. She was the longest-living and least pleasant of the two. Isn’t that always the way?
My dad’s mum was “dadhima”, and my mum’s mum was “nanima”
I only really knew one of my grandmothers – she was the Southern one. She always insisted on being called “Mammy” but for reasons of a personal nature, that term squicks me out and she was always “Gramma Sloan” to me. My MIL insists on being called “Mammy” and yet, is just “Gramma Lois.” My daughter’s paternal grandmother is “Gramma Phyl” – think there’s a pattern here? My mother would be known as “call the police, and grab the shotgun for me, wouldja” if she ever darkened my doorstep, but I doubt that would be a good name for the OP 
Of course, I do not want to ever be a grandmother, but if it happens, I will insist upon being called either “Grandmama” or “Grandmother” – or maybe even “Great Evil One” I haven’t yet decided, but my children have informed me “Cthulhu” is taken.
ETA – my husband’s grandmother was “Granita” – which happened when a bunch of little boys (my husband and his brothers) mixed together “Grandma” and “Juanita” – I always thought that was adorable.
My grandmother is Hooa (WHO-ah) to my sisters and I, and Oo-Hoo to my cousins.
We think it’s because she always announces her presence with “Yoo-Hoo” instead of “hello”.
Clearly the names are ones the we and the cousins came up with ourselves after we started verbalising, I have no idea how our parents referred to her before that.
Can I be a bit eccentric and suggest you wait 9 months and see what the baby feels like naming you?
May I suggest it’s not about you, it’s about your grandchildren.
Be happy you have a healthy family - and go with grannie, grandma, nana or similar. It’s not ‘country bumpkin’ to use these either - it’s what a kid can manage to say.
Ours were both Grandma Firstname. The great-grandmothers were Grandma Lastname. Grandpa was just Grandpa because there was only one of him.
My Dad’s Mom took a shot, I think when I was about five, at convincing us to call her Mimi or something, but Grandma had already been established. Looking back, years later, it had probably been provoked by her sister, who had successfully pushed Mimi on her family.
Children do not like to change. As a compromise, we offered, in all innocence, to call her Granny instead. This was not met with joy. She went on and on about what the word Granny drew to her mind. She dropped the push for Mimi. My Dad had a few words about foolish vanity after she was gone, to let us know that he wasn’t upset with us for upsetting Grandma.
I, too, called my grandmother, baba. Sometimes baba-love. 
I miss her.
Oh, and my grandfather was poppy. Miss him, too.
My mom is Nani/Nonny to my kids.
They call their other grandmothers Nana and Grandma.
I called my grandmother by her first name, because she had a funny quirk about titles. I called her mother by her first name too–it was an inherited quirk.
My kids call both grandmothers Grandma, with the last name added on if we’re trying to differentiate. Usually plain Grandma means my mom, who lives here in town.
My mother insisted on “Meme” (pronounced “meem”), though where she came up with this, I don’t know. As she was a lifelong Harrisburg resident, perhaps it’s something you should use also 
Some suggestions here (especially if you go to the “other languages” link)
http://www.namenerds.com/uucn/granny.html
Damn, Dangerosa, that breaks my heart for you. My fondest childhood memories revolve around my Grandpa, and I’m so thankful he loved being a grandparent.
I became a grandmother at the ripe old age of… 30. Yep, that’s right. 30. I married young, and to a wonderful man who already had 2 boys. I didn’t want the boys to feel like they had to call me “Mom” or “Mommy”, as they had a mother - and a father - who loved them very much and was a part of their lives. I later came to realize that I would have really liked to have been called a nickname of some sort, but hey, I was young and was trying to do right for everyone involved.
Fast forward 11 years. Our oldest boy, at the ripe age of 17, finds out that he and his girlfriend are expecting. One night, after a long conversation, I asked him, “I would like to ask one more thing. Who will I be to the baby?”
He didn’t miss a beat or bat an eyelash. “Who else? You’re ‘Grandma’, of course!”
And I have been one of the proudest Grandmas ever. I revel in it. Yes, he knows I’m not his biological grandmother. He told me so. He also informed me that he didn’t care, that in his heart, I am “Grandma” plain and simple. No one can tell him otherwise.
Oh, and we both love the looks on everyone’s face when we’re out in public and he calls for me. Sometimes, I swear he does it on purpose just to see others’ reactions.
My maternal grandparents were Nana and Pop. My paternal grandmother was Grandma Lastname. By the time I was four there was only Nana.
My husband’s grandmother didn’t want to be called Grandma because it made her seem old, so she wound up being Stellamom, and his grandfather was Grandpafather.
We have just acquired three grandchildren, one by birth and two by marriage, so we’ve sorted out this question recently, too. The two older ones already have a Nana and Papa, so that was out. I could do with Grandma, but I didn’t want to be Grandma Lastname, since that was my late MIL. So we looked things up on the internet and sent lists of possible pairs to my husband, who works out of town. I was kind of leaning toward Lolli and Pop, but he chose Oma and Opa. It works, since my ancestry is mostly German. It’s also easy for the kids to say and remember. Up to the time we told them that, they were still calling us Stepdad’s Mom and Stepdad’s Dad. Or mostly nothing. As soon as we told them they adopted it like they had just been waiting to be made official. It’s been about six weeks and you’d think they’d never done anything else.
I think it will also carry over to any others who might come into the family at some future point, and that’s sort of what I wanted.
Mugga was my grandma. My oldest brother got confused between mother and grandma, so he settled. She was the only grandparent I ever had.
I was absolutely thrilled when my oldest son out of nowhere called my mother in law Mugga. Now she is mugga to my kids and my nephew. I may have encouraged it a bit.
Moma.
My grandmothers were Grandmother and Meemaw. As it may sound, I had the more formal relationship with “Grandmother” even though I saw her a lot more.
When my niece was a baby my sister-in-law asked my mother, “What do you want her to call you? I gotta tell you my mom’s already down for Grandma, but she can call you Meemaw or Granny or Nanna…”
My mother: “How about Blanche? It’s my name.”
Sister-in-law: “Well, she needs to call you something other than that…”
My mother: “Well then… I’m partial to Größmama.” [not sure if it’s umlauted]
Sister-in-law: “That sounds a little Nazi.”
My mother: “Then we’re back to Blanche.”
Sister-in-law: “Well there must be something else you want her to call you.”
My mother: “She can call me Long distance. But not collect.”
Which is why my mother’s grandchildren called her Blanche.
My grandmothers were alway Nanny for my Mom’s Mom and Nanny my last name for my Dad’s.
This works great as my Dad’s Mom is called Nanny by my kids and my niece and nephew and my Mom is just Grandma and my wife’s Mom is either Grandma or Grandma Judy.
Dad’s parents were Grandma Firstname and Grandad. Grandma Lastname was Grandma Firstname’s mom. Grandad’s mom was also a Grandma Lastname.
On my mom’s side I had Abue Firstname (my Mom is Mexican). Never met him (he died about ten years before I was born), but my grandfather on that side was always Abuelito Firstname. Abue Firstname’s mother was called Toti; I think one of my uncles named her that. My cousins weren’t allowed to call her that because my aunt thought it sounded silly.
I know it’s hard, but if it were me I’d just go with Grandma or Nana. You’ll grow into it. Grandmas are awfully young these days.
GT
Mine were both Grandma Firstname. Easy enough.
I have to chime in with the same advice others have given…don’t worry about it! I think it’s awesome that you’re such a young grandma, and whatever word you use doesn’t change that fact, anyway. I’m exactly your age, and my kids are still babies. One of my fears regarding being an “older” mom is that I won’t have a chance to really enjoy being a grandparent. So…embrace it! Don’t let a silly thing like a title take the fun of it away from you.
My paternal grandfather died before any of his grandchildren were born. Consequently it’s always odd when referencing him- rather than “Grandfather” (since he was married to Grandmother) he’s always referred to as “Daddy’s father” or “Grandmother’s husband”.
When my Number One Nephew was born, my mother was scandalized at the thought that she wouldn’t be “grandma”, since my SiL thought that having two grandma’s might confuse the tyke. (this, despite my brother pointing out that he and I did just fine with that.) Ended up, my mom is “grandma”, and SiL’s mom is “CeeCee” – for reasons I don’t quite understand, and suspect have to do with “CeeCee” not wanting to think of herself as that old. They all congratulated themselves, repeatedly, on solving this Gordion knot of a problem.
And then, three years later, Number One Nephew decided that everyone was going to be called by their [Firstname] – whether they be mom, dad, grandma, or ceecee. Consternation and alarm, all around!
Particularly when they realized that I was still Uncle [Firstname], just like I’d always been.
Number One Nephew eventually grew out of that phase, but when Number Two Nephew came along, there was no more nonsense about who wanted or did not want to be called “grandma”. You take what you get, namewise, and you’re happy with that.
OP, be warned.