You might ask your vet to do a blood test to check for the eosinophil levels. My cat Lucia licked all the fur off the back of her front legs and the insides of her thighs, she turned out to have eosinophilic dermatitis, like an allergic reaction, that caused her to start licking, then become a bit complusive about it. A short course of prednisone tablets fixed her up and she never had the problem again. My little ginger Sammy has it, it manifests as small red spots on back legs, and his bottom lip gets a bit swollen.