Why and how does sleep paralysis happen?
There are chemicals in your brain that stop your body from acting out your dreams of say, you dream you are running, fighting, etc. What happens in sleep paralysis is that that chemical is still working even though you're awake because your body hasn't registered you're awake yet. You hallicinate and can't move for a few minutes and it's scary but completely harmless. It happens to everyone at least once in their lifetime. I'd only be concerned if you were experiencing it regularily.