Amputation / nerve damage

I'm 26 years old and am otherwise in find health.  I was born with one hand as a result of amniotic band syndrome.  My left arm stops about four inches below my elbow.  I've recently resumed cycling again, albeit on a new bike this time.  When riding (frequently long rides, three or so hours) I rest my left arm on the handlebar such that the bar rests on the notch between the ulna and radius bones at the tip of my stump.  After my last ride (about two days ago) I've had a small patch of skin near the tip of my stump that's about an inch long and half an inch wide that is numb, much as though it had been numbed with Novocaine; it can feel pressure when poked, but is much much less sensitive than the surrounding skin.  It's like your cheeks when the Novocaine is starting to wear off at the dentist.  

Do you think this could have been caused by my riding position and, if so, do you think it will heal if I alter riding positions?