My cat has been ill for some time and has a long history of bladder infections. Each time after he has been given antibiotics and pain medications he gets better. Lately I have noticed that his back legs have lost a lot of muscle mass, his legs are weak, and he moans when I touch his flanks. What is wrong with him?
It sounds like your cat has a urinary tract infection that may not have completely resolved with treatment. We would not expect for him to get better and then have muscle weakness/muscle wasting related to a urinary tract infection. It may be related or may be a separate concurrent disease process. Either way, to sort out if they are related or not or just occurring together, further tests are likely necessary for your cat. He may need radiographs, an abdominal ultrasound, and some blood work; the first step would be to go back to your family veterinarian and discuss your concerns. Your cat may need to be referred to a specialist (an internal medicine specialist may be a good first point).