It is common to develop skin issues after having had fleas. DERMA-IONX can help with the allergy response. Cats can develop an allergy to flea saliva and also acquire tapeworms from the fleas. If you have not already, bring a stool sample to your veterinarian to check if your cat needs to be wormed for tapeworms. Besides DERMA-IONX I would also give PROBIOTICS and OMEGA 3,6,9 to restore the gut and reduce inflammation. This usually improves the skin and coat. Be sure to eliminate the fleas. The majority of the fleas are in the environment and not on the animal, so you could have a few hiding out and jumping on for a quick blood meal. One flea bite can create an allergic response.
Please let us know how you do. Sometimes altering the diet can be helpful as well and we can advise you on that if necessary.