I have a rat terrier that is 15 yrs. old. She scratches and bites herself all the time. Two vets have told me?

it was allergies. She did take shots but they quit helping her. I give her benadryl. She has a funny smell so I wonder if it is allergies. She's also losing her on her backside and skin is red. Anything I can try at home that might ease her itching?