My best guess is a pinch nerve somewhere in your back. Numbess is usually a result of a nerve impengement or sometimes excessive fatigue. Since you were doing good mornings which doesn't put much strain on your arm, it is probally in your back.
Is there any pain, tingling or loss of sensation or strength?
How long after your workout did you feel this? Did it last?
Does anything else hurt/feel weird?
Any deformities, abnormalities in your arm or back?
These questions will help narrow it down.
Try dropping a line to Dr. Ryan, he'll know much more on this. As always, internet advice cannot replace a physicians care. If symptoms persist for more than a week or two, get checked out.