Yep, its sleep paralysis. I had one event only.... thank heavens for that! It's extremely frightening. I dreamt I was laying in my bed and a creature was sitting on my chest and staring me right in the face. I wanted to scream but couldn't. I couldn't move either. It was like it wasn't a dream at all.

My ex woke me properly and asked what was wrong as he could hear me 'moaning in pain'.

There is lots of info on the web about the sleep disorder.