There are several possible causes of weight loss or gain in cats, including medical conditions and overfeeding/underfeeding.
As the cause can be caused by a disease or dietary, it is important that you seek the advice of your veterinarian to determine the cause. The vet will do a complete physical examination, obtain a medical history from you, ask questions about diet, what food, how much, how often, and possibly take blood and urine samples for testing.
