It could be a skin irritation-- sometimes they will chew their hair off if their skin is very itchy (could be fleas). My cat was doing this and we found that it was due to an allergic reaction. By switching her to a limited ingredient diet, we were able to lower the number of allergens enough to keep her comfortable.
it could be that he/she is excessively licking the stomach for one reason or another. if they lick one particular spot too much, the hair will begin to fall out and stop growing.
Your cat could have a rash, or some sort of allergy to something in the house, the food he eats, or anything else that might cause trouble with his skin. It could also be a mite or fleas that will do that. Another one is ring worm that could start small and grow. Go to the vet.
Hair loss has many causes, all of which need to be determined by a vet. It could be as simple to treat as an allergy or a parasite, or it could be as serious as Cushing's or Addison's Disease, or a thyroid problem. Lab tests will tell you.
If your cat is a female and isn't spayed she could be pregnant. When cats are pregnant they lose hair around their nipples. Another possible explanation is that your cat is allergic to something it's coming in contact with and that's causing the hair loss.
Well that may indicate a loss of protein in her diet. So maybe there also might be something else that is wrong with your cat. You will not know until you go to the vet.