I know exactly what you mean Leigha, it’s not a nice feeling at all!
Basically when you’re in a deep REM sleep (the deepest part of sleep during a night), your muscles go into a special state of stiffness called atonia. This means that there are no brain signals going to these muscles, so you are actually paralysed while you sleep! This is to protect yourself so you don’t try and act out your dreams. But if you wake suddenly from a dream or because of a noise, your brain might not catch up so quickly and your muscles stay paralysed, only for a few seconds or so. This disconnect between your body and brain gives you a sensation of falling, so when then once your muscles kick in, you jerk out of the bed!