Sleep paralysis. Yup.
Its basically you wake up in the middle of your sleep, but you can't move at all because your brain pumps this one fluid thingy thru your body that stops you from acting out your entire dream in your sleep. So if not being able to move at all isn't scary enough, you start seeing these weird sort of foggy figures about to do something to you. And you can't move to defend yourself at all. You just helplessly watch as they do whatever your having the nightmare about. Pretty scary stuff.