This is what happened to Inca a while back. Her's was on her backside, the vet said that she must have got bitten as she was trying to run away from another cat. She was shaved, (which did look kind of funny) and put on antibiotics. She felt very sorry for herself afterwards, and after I got the bill from the vet, so did I.
They do look funny with the shaved rumps. I know the feeling about those damn bills!