man... i have this kind of thing... and it also started to happen after i started to try astral projecting/meditation... for me happens mainly when im sleeping without being sleepy (just for laziness), and am having lots of dreams. Its scary, mainly when you sense a 'presence' near you, and cant move... Then you kinda 'awake' , move, and this feeling of the 'presence' of someone/something vanishes...
Anyway ive read somewhere that this feeling of sleep paralysis can be used as an way for inducing astral projection. If i remember well, whenever you feel you were awake but couldnt move, you should try to imagine the feeling of being floating, until you separate yourself from the physical body. I never could try this, because i never remembered it when the sleep paralysis happened with me, but i think it would be nice a try.