I used to find it happened only when I was lay on my front. But just the other day I had it when I was on my back. I've had it more when I've been extremely tir

I used to find it happened only when I was lay on my front. But just the other day I had it when I was on my back. I've had it more when I've been extremely tired and finding it hard to sleep. It's only happened a handful of times in my life that I can recall, but it's really unpleasant. It feels like you are trapped, can't move and have to will yourself free.