There are two possibilities that come to mind. We had two cats with hair loss problems. One had allergies. She had hair loss on her lower belly, which was licked completely pink because, presumably, it itched, poor girl. We didn't know what she was allergic to. We never could find a way to treat it, but she didn't scratch or scab so we just left her alone. The other cat who had hair loss had a nervous habit of licking certain spots too much so that the hair wouldn't grow there. It happened suddenly, just after my son was born, and she never got over the habit. Because she licked around her ankles and sometimes around the end of her tail, she looked a little bit like a trimmed poodle at times. At other times, the hair would grow back to almost normal. Your vet could help determine which might be the cause, or if perhaps something else is involved - diet? New meds?