Talk with your doctor. I think this might be as Rikki mentioned something subconscious, similar to a placebo. It happened once or twice on coincidence but you've convinced your mind it will always happen. You doctor should be able to help you with sleep paralysis.
Regarding your spell casting, if you're using your bodies energy and not drawing extra energy into your body this could explain why you feel bad after casting, so you may wish to do some basic energy work to check you're charging/grounding properly, but I feel the sleep paralysis is a different issue you should look into medically. Good luck to you.