Originally Posted by Hazmat
Might just be mites or lice. Best way to treat is to use Frontline or Revolution (Advantage will not kill them). The cheap way is spray her with cheap store bought cat flea spray that has pyerthin as the active ingredient.
The Op posted this "Her belly has gotten droopy (hanging) but she hasn't gained any weight" . I don't think mites or lice would cause that to happen.
The cat is drinking a lot of water and losing fur, it sounds like there some kind of medical issues going on.