Ok so Tinkerbell is allergic, and she itches alot

Well about 2 1/2 weeks ago I took her in to see the vet and she was given a shot and some meds for itching

, She got better for a few days and now she is itching again.....I even switched her food to Wellness, Duck and Rice,........I read it was good

She has been on this food for about 2 weeks now and no improvement, she was on Nutro Ultra which I now know it's not good

......................My vet called to check on Tinkerbell today and I told him there was no improvement expect for the first week or so and he said to take her in tomorrrow to put her on a special diet

and he said I must refrain from giving her ANY treats of other foods

HOW AM I SUPPOSED TO DO THAT??? But the main quesiton is this?? Do ya'll think is ok to switch her food again......that would be 3 different types of food in 3 weeks

Thanks, Mickie