I need help - cat urine burn?????

by Judy
(Illinois)

My cat has no hair on one of her hind legs on the inside. The skin is red and raised like hives in the pattern of a dried river bed. It has been described to me as urine burn, or possibly an alergy to the litter in the litter box. Litter type has not been changed. What would cause a urine burn? Can you give me any answers? Please