One of my cats is going through the same thing as well. She lost the majority of her hair on her lower back. My vet put her on antibiotics and a steroid medication (prednisone) and gave her Frontline. She does have fleas so this more than likely is a flea allergy. You may want to consider giving your cat flea medication (advantage) every 30 days to see if it helps/or takes care of the problem. I know it can be bothersome to see our kitties constantly licking and scratching themselves.