Don't worry, it's a harmless condition called sleep paralysis. I used to get it when I was a lot younger, it was terrifying, I thought I was dead. I tried to move or scream for my Mom to come into my room but couldn't, it's like your mind is awake but your body isn't and you can't move for a few minutes.

I haven't experienced it or thought about it in several years, your post brought back my memories of those frightening episodes.