It’s better for your cat to be thin then to be too fat.  It may be that your cat is still growing and that’s why it seems to be thin.  Veterinarians say that for a cat to be healthy, you should be able to feel its rib cage but not see the bones sticking out and feel a few of the vertebrae in its back through its fur.  If your cat continues to eat very little, and continues to loose weight, you should make an appointment to see your veterinarian right away.  Cats can get serious illnesses with very few symptoms.  They can get sick from things that are in their environment, such as pesticides that may have been applied to a green area where they spend time, or from mosquito bites.  Also, cats do get diabetes, and forms of anemia.  Don’t worry, just be vigilante, like you are right now.