Sounds like a crush injury. Adjust your grip so the weight is mashing into the pad of the last segment of your thumb and not higher up at the joint or bone of the segment above it.
If you aren't taping and/or chalking, try them. A little more efficiency with the grip may help you wiggle into a little better position.
If you have access to different bars, test them. Differences in diameter may be enough to get pressure off the nerve.
I once had 9 months of numbness after a single afternoon using a repeating Hamilton syringe in a lab because my hand had gotten tired and I'd started depressing the button by pushing at the joint line.