It's happened to me quite a few times. The most recent one being a few days ago. That one was a bit startling. I felt like something was sucking the life out of me while I sunk into my bed. I could hear myself screaming in my head, but couldn't move (of course). I kept telling myself that everything is okay it's just sleep paralysis and tried to wake myself up. I felt better after a few seconds although I was still a bit shook from it. I noticed that I usually get sleep paralysis when I'm super exhausted.