That's sleep paralysis. It's a common experience, typically experienced once or twice by the average person, and certain things can make it more likely to happen, like attempting lucid dreaming. I've also been attempting lucid dreaming and all I can say is if it ever happens again DON'T OPEN YOUR EYES UNTIL YOUR BODY IS UNDER FULL CONTROL, try moving little bits like toes and fingers first(like in kill bill), and realize that your mind is playing tricks on you. You probably already knek this.