This happens to me on occasions as well, but it obviously doesn't disturb me as much as it does you because I pretty much just ignore it since I'm used to it.

Just try to remember that you cannot fall (unless you roll out of bed) and that the sensation is just your body messing around with you basically.

Some scientists think that its an evolutionary throwback to when we used to live in trees and that its a defence mechanism to make sure you have a firm grip on your tree but who knows?

Incidentally; because of this you should have a good natural ability to go straight into lucid dreams; if you find yourself lying in bed and then the next thing you know is your completely somewhere else congratulations you're lucid dreaming. (I literally just find my mind slipping and then im doing something else half a second later)
(just make sure you dont go "WOW IM DREAMING" and wake yourself up ok?)