could be caused by most anything, with the most likely cause irritation as a result of allergy, or of mites (mange).

Cushings has a multitude of symptoms of which the most usual are an increased thirst and urination, loss of weight, thining of the skin, muscle weakness including that of the heart along with high blood pressure. It can be artifically induced by the administration of large amounts of Prednisone.

You need to obtain some antibiotics from the Vet. While there have them do some skin scraping to check for mites.