Something similar happened to me a couple of times. Now I already get used to it but the first experience was quite terrifying.
I woke up in the middle of the night from a lack of air and just felt that there was someone outside. It didn't move, it made no sound, it did nothing. It just was there - a black creature behind the door watching my weak attempts to break free and begin breath. And this feeling of helplessness was perhaps the worst.