I get that quite a lot; it used to freak me out a bit too. Makes you wonder sometimes if what you're doing is actually a dream and you're going to wake up, a bit like being in the Matrix or something...

edit: If you do it again do what I do, try doing something that would normally be impossible; that way you'll know you're still dreaming. You can actually have quite a lot of fun with it, like jumping around and climbing up the walls, again like being in the Matrix. Manipulate the situation and have a laugh. I like to leap out of my window and float to the ground; but I always do a test first in my room to make sure I'm not actually awake, though when you wake up properly there's never any real doubt.