pepper, wasn’t it around the beginning of December that you were accepted to college?
Now, I’m no expert, so I’m just going to start pulling some ideas out of my butt, okay? I’ve got experience with clinical depression, SAD, PMS, and a bunch of other loss-related stuff that may be of help. Or you can tell me to shove them back into my butt, where they belong. 
Not only is it the holidays, it’s your last Christmas as a “little girl.” This time next year, you’ll be all grown up & in college. That first holiday where you can make your own choice as to what you’re going to do is liberating for some, traumatic for others. But the realization that it’s going to happen can be a shocker as well.
Post-holiday stress can also be a factor, as others have said. The big buildup, then the one day, then…it’s over. I think that’s a very large part of the reason why couples take a honeymoon after the wedding. Planning a wedding is stressful, and once that One Big Day is done, you need to relax. Christmas can kinda be the same way.
Also, you mention that you haven’t had a period in quite some time. Could be that your hormones are seriously out of whack. I would definitely see a doctor about that. If you’re not pregnant, and there’s nothing wrong with your reproductive system, you should be having periods approximately once a month (the time frame does differ from woman to woman).
These are just a few ideas that popped in to my head when I read your OP. As I said, I’m NOT an expert, so don’t take them as gospel.
Hope you get it figured out soon, though! ((((pepper))))