What causes sleep paralysis?

A long time ago I used to get sleep paralysis when I slept on my back. That’s not the reason anymore.

Now it’s when I wake up early in the middle of the night and go back to sleep I always have sleep paralysis whether I sleep on my back or side I always wake up early for some reason and when I’m sleepy and want to go back to sleep I just have this sleep paralysis.

Ps. It doesn’t happen when my bf sleeps with me though. It rarely happens.
It’s creepy as hell. I used to get nightmares that come along with it. I don’t as much anymore but I feel like it’s more of a dying experience and that I’m losing myself and if I don’t fight harder to move and open my eyes I’m gone. It’s really scary. I try to wiggle my fingers and I feel it shaking but I can’t wake. It seems to be getting worse/stronger.

I read online it might be a vitamin deficiency of B12.