it's called Dupree's contracture my father has it. It hits mostly northern Europeans and is scar tissue that builds up on the tendons pulling them inward and tighter (claw hand) and can re-occur. My Dad has had the operation 3 times. He was a mechanic for 30+ years and handling the cold tools over the years apparently was one of the contributing factors.
Yes can get very painful! Hope you get it taken care of soon