There are a number of possible reasons for this to happen, though really only a doctor can diagnose you. I would look into taking glucosamine pills for now to help with
joint pain and see if it helps at all. I'd make another doctor's appointment to get your hand looked at and see what they say. It could be from a genetic condition, from trauma, or from a malformation in growth, among some other possible reasons. I hope you can get some answers soon.