Sounds like what happened to my Brandy. I switched her food and she itched constantly for a week. No bite marks so I was certain it was the food and changed her back. She was still scratching. Took her to the vet and she had mites
She was given a shot and 2 days later the scratching stopped. I have always used Revolution on her for flea treatment but it needed a prescription. Last month I bought something different at Petco, thinking it was basically the same. Huge mistake, it didn't control mites, which Revolution does. From now on I will only use Revolution. Good luck and hope it will be an easy problem to solve.