It takes more than just lying back and concentrating to P-shift, Erin. Those who can P-shift are normally born with the ability to do so and forcing oneself to do it without having a natural affinity for it takes years of meditation, self-hypnosis, and training.
The only explanation I can give you for why your hands went numb is this... When one concentrates long enough, something will happen. Not always what the person wanted, but something usually does happen. You probably just started to slow your heart rate and breathing to a point where you were about to fall asleep.