Please see your veterinarian. The skin can only respond to problems in a limited number of ways and cats can react to those issues as well. Having an exam and simple hair/skin tests will likely reveal the problem to your veterinarian and thus to you to get your cat treated well. Fleas, mites, allergies, other parasites, bladder problems, skin infections can all be factors in what is happening in your cat and how to treat it. It is highly likely that one or more of these problems is the culprit that needs prompt veterinary treatment. Your cat is obviously bothered by the situation and your veterinarian can get your cat some relief.
I agree with Dr. Amthis, veterinary evaluation is needed. Behavioral causes of self mutilation are possible, but are diagnosed by eliminating other treatable medical causes such as parasites, bladder pain, spinal pain, etc.