I know this sounds stupid. But I just got a pool this year. I have ALWAYS been around water. Tried to climb out of the boat at 5 years of age. Anyway, tonight I was in the pool (about 4’ deep) and just relaxing, I started floating. I had to actually try to not float… not try hard, but if I just relaxed and layed back a little my knees immediately rose to the surface. What is up with that!!! I was in the pool yesterday and this was NOT the case. Anyone have any answers??? I am wierded out about this. My right side has felt like someone stuck a sock under my ribs for about 8 months now, so that is probably why I’m freaking out… don’t know.
I just know getting older sucks!!!
Had a bit of gas? Just try farting next time and see if you sink. 
I thought all humans float naturally, because the human body is less dense than water.
Did you used to sink like a rock?
If your definition of float is very broad maybe. I for one don’t float by most definitions of the term. Most certainly not “knees out of the water” like the OP. That would freak me out too!
If I hold my breath and go limp I will be underwater (vertically) with the top of my head bobbing in and out of the water with my nose never coming up. If I let go of my breath I sink to the bottom alarmingly quickly.
Fat is less dense than muscle.
Just sayn’…
This was true for me as well up to about 10-15 years ago. It is alas no longer true for the reasons given in the post after yours.
a relaxed body is less dense then a tensed body. on your back it is easy to fill your lungs with air.
Back when I played soccer and was in insanely good shape, I could jump in a pool. Then, if I exhaled all the air out of my lungs I’d sink like a stone. Right to the bottom. Muscle and bone are more dense than water.
These days I’m a fat bastard so I expect I’d float like a cork. Haven’t been swimming in a few years, though.
Everybody saying “fat is less dense” might have missed this part of the OP:
Same here. Even when I wasn’t in particularly good shape.
When I was in college and in pretty good shape, I could sink to the bottom of the pool even with my lungs full of air. Before and after, not so much. I actually thought it was cool to sit on the bottom of the pool for a couple of minutes, since I’d read Stranger in a Strange Land. The lifeguard had a different opinion.
I actually have a hard time floating in the water at all. Even relaxed and on my back I have to kick a little bit to keep my head above water. If im vertical i just sink to the bottom lungs full or not (much quicker without air in my lungs though.) This only happened in the last few years. I always used to be able to float and frankly its much more difficult to swim now XD
You are getting fatter.
Muscle is more dense than water. Fat is less dense.
I am near neutrally buoyant. If I hold my breath I float. If I exhale I sink.
I had a roommate in college who took second place in the collegiate Mr. America (or something like that…I forget). He could hold a kick board and jump in a pool and sink. He was a BIG guy, near pure muscle and amazingly low body fat. He sunk like a rock. He simply had to swim if he wanted to survive in water.
Personally, I cannot float on my back. My legs sink and the best I can do (holding my breath to float) is ending up upright in the water with the top of my head only bobbing on the surface.
In the end floating is about displacing water. It is a balance between water you displace and the weight of the mass displacing that water. This is why ginormous steel aircraft carriers can float while my former roommate sank like a rock.
To reiterate, from the OP, again,
I doubt she got fat enough overnight to float when she couldn’t before.
My guess would be that yesterday you were unconsciously positioning your body in a way you hadn’t before. Floating is partly a skill, like riding a bike or whistling; in trying and trying, sometimes you just suddenly hit on the trick.
Even lungs full, I sink. It just pisses my wife off to no end as I am not a small being (6’2 270lbs), but I haven’t really been able to float on my back since I was 10-12 years old. I unambiguously sink and treading water is incredibly exhausting and there is no way I could do it for a very long time. If I hold my breath, I will eventually end up lying flat on the bottom of the pool (and I have had at least 3 lifeguards jump in after me which I view as a proof that sinking is not normal behavior). Now put me in the ocean and I love it beyond all else as I can be a normal swimmer!
So in answer to the original question, it would freak me out too if I could suddenly float. I would view it as having put on an incredible amount of fat.
editted:
It looks like Joel Upchurch had a similar experience wrt extended stays on the bottom of the pool.
It’s helpful to think of buoyancy and floating as two separate things.
Buoyancy comes from how much water your body displaces – density, basically. Women are usually neutrally buoyant. Men are usually negatively buoyant (they tend to sink). How much muscle or fat you have affects your buoyancy.
Floating is a skill that can be learned. There are things you can do to affect your buoyancy in the water – holding air in your lungs (lowers average body density), body position (easier to float laid out than upright). These are the things you learn to do when you’re taught to swim. You might not be conscious of doing so, though.
It was probably your body position that changed your buoyancy and made you float or sink. Try this experiment: float on your back laid out as flat on the water as you can manage. Then, without changing how much air is in your lungs, change your body position so that you’re more “sitting” in the water (butt down, knees closer to your chest). At some point, you’ll find yourself sinking down. Although, probably not “like a rock”.
And as to the feeling like a sock under your ribs: may be a pulled or strained muscle. I get that, from time to time; doesn’t hurt exactly, but feels like something is there that pulls when I move in certain ways. Don’t stress it with exercise for a week or so, and maybe taken an OTC antiinflamatory. If it persists, have a doctor check it out to rule out something worse.