My daughter's lab did this and it was a reaction to the flea/tic medication he was on. He had been on it for several months before this reaction. It must have been an accumulative thing. They discovered it by accident as in final desperation my daughter stopped giving him anything including the flea/tic (did give his heartworm) and by the time the med was 30 days passed due, this had stopped. He had licked so much he also lost the hair off his butt. Don't know, but sometimes you can take a med with no problem and develop an allergy later. Strange thing was, it only seemed to bother his butt. No problem since as long as he is not given any flea/tic med of any kind (so far so good - no fleas or tics either).