This sounds alot like Erin's astral experience in her blog, Astral Projection: My First Experience
I've had those kinds of things happen to me before, it's like your frozen can't move and there are other things there, or you're alone. Either on is very scary.
It's possible you're close to astral projection, I don't know I just think that's what is happening to me.
Question for you, is there a light, or a tv on when you're sleeping and having these nightmares?