Hmmm....I wish I knew what food it was

It probably IS the food that is causing this. Is it something that you BOUGHT from your vet? Or just a food that he suggested that you picked up somewhere else? And if you bought it somewhere else, where did you buy it?
It's not Science Diet is it? Not Eukanuba or Royal Canine? Do you remember what the bag looked like or what color it was?
Sorry for the 20 ?'s LOL