This has only happened like 5 or 6 times but it scares me everytime it happens. Right as I'm about to fall asleep my bed will start to shake as if there's something under it but my bed is on the floor. I'm not dreaming because I know I'm awake when it happens, but I'm afraid to try and fall asleep now. Has this ever happened to anyone or anything like this?

Why exactly where you thumbed down I wonder?
Actually, thinking about it again, one of the times I had it turned out to be a very minor earthquake (which are happening world-wide pretty much all the time). Maybe some people are just more sensitive to them.
I looked it up online after that and found similar stuff under symptoms of vertigo. :3
it was either that or believe there was something under the bed. lol
Check your bed to make sure that it was properly put together and that the screws are tight. If this isn't a problem, it could be due to a construction flaw in your home or paranormal activity. If you suspect that it's paranormal activity, contact a professional psychic and see if he/she can get the "being" out of your house.
Good Luck!
Its your own body 'falling' to sleep. You just tend to shake a little, like you are shaking off the day. The only thing is its done subconsciously so you THINK the bed is shaking, when its NOT.