Izzy, I have good news for you. What you're describing is very normal, very common, and very benign. There are different names for it: hypnic jerks, hynagogic jerks, myoclonic jerks, myoclonus, etc. I've experienced it many times. Sometimes people get the sudden feeling that they're falling, which causes them to awake with a startle. It's just one of those weird things the human body does that you don't need to be concerned about.