It's a hypnagogic myoclonic twitch.

Basically, you've fallen asleep and a muscle, probably in your leg or arm has twitched, strongly enough to wake you up. The springs in your bed respond to the sudden movement giving you that bounce, and your mind interprets it as having fallen back onto your bed.
It's quite common and perfectly normal. I've often had it with a dream in which I've tripped or stumbled, reinforcing the feeling of having fallen into bed.