You might want to ask the vet about eosinophilic dermatitis (a mouthful, I know, lol). My cat Lucia, who passed away last winter, had suddenly (several years ago) started overgrooming her lower tummy and the insides of her front legs, to the point they became bald! She was indoor only, so flea allergy was a non-starter. After some thought and tests, our vet did a blood test, where they did an eosinophil count, and it was high. A short course of prednisone calmed it down and it never recurred. I often see folks ask about overgrooming and skin issues, and I'm surprised more vets don't think about the possibility of this type of dermatitis. As I recall, it's like an allergic or immune reaction, and could be brought on suddenly by any number of things.