Telltale signs that indicate a possible grain allergy include excessive itching and scratching, endless grooming, hair loss in patches, flaky or red skin, scabs or grooming injuries, watery or foul-smelling stool, noticeable gas, very bad breath, and vomiting. If your cat has a grain allergy, switching to a single-protein, high-quality grain-free cat food should result in noticeable results in a few weeks.