[Ellie May]

Could be so many things and if the vet can't figure it out, you may need a derm. But I've been taught that when it is toward the tail, it's flea allergy dermatitis until proven otherwise. It's less likely because you are using a flea preventative, but sitll possible. Have you looked with a flea comb? The flea drugs are becoming resistant. The antihistamine may help depending on what the problem is.