I have a 12 year old dachshund mix who I’ve had on grain free food for about 5 years. He has had ear issues over the years but it’s been manageable and infrequent. I have been transitioning him to Purina Pro Plan for bright minds and all the sudden his ears are bright pink and he’s shaking his head constantly. I have read so many terrible things about the grain free diets but now I wonder if I made a mistake and he might actually have sensitivity to grains bc I had him on grain free food for so long. Is that possible? Should I just go back to the grain free food given his old age?