When I was in the Army it happened a few times. I was always aware that I was awake and unable to move. I definitely felt a presence in the room with me and that was very unnerving.
It might have been stress related, but the odd thing is that I've never had sleep paralysis when someone actually was in the room with me (my roommate in the barracks or when we slept in tents while in Bosnia). It only happened when I lived in a room by myself.