There is sweet kitty breath and then there is rotting tuna breath. Bad breath in cats, as in humans, can stem from a number of different causes that can be either oral or systemic and sometimes both. Chronic bad breath can be a sign of a serious oral infection, rotting teeth or infected gums. People go to the dentist regularly, but cats don't. Doing some detective work, testing the tried and true fixes, monitoring your cat carefully and consulting a professional when needed will allow you to zero in on a solution that brings back the sweet kitty breath.
Provide a constantly refreshed supply of water in a clean bowl. As with humans, when cats are dehydrated, their breath will smell. Cats are not naturally big consumers of water and their sense of smell being so superior to ours, their notion of fresh water is actually much more refined. A surefire solution here is to invest in a cat water fountain that keeps the water moving and filters it constantly, both of which attract the cat to drink more water and more consistently.
Alter the cat's diet to include at least some dry food or kibble along with wet food. The dry crunchy kibble works as an abrasive to clean the teeth and mouth. If your cat is on an all wet food meat-based diet, just add some good quality kibble to the diet.
Use a cat toothbrush and cat-safe cleaning paste to clean Kitty's chompers. This can also be done at a good animal groomers or veterinarian if you are not up to the task yourself. This will also give you a good look into the cat's mouth and you may be able to judge where swelling, loose teeth, redness or pain response is coming from in her mouth.
Visit the veterinarian for an oral exam. If there is an infection or a diseased tooth, the vet can easily pull the tooth and treat with antibiotics. Many serious oral disorders can be treated effectively with medication, but most do not improve with time on their own.
Maintain good oral health by giving treats and snacks that are oral health enhancing. Provide a supply of fresh cat grass to chew on and eat, which has breath-freshening chlorophyll. Give your cat treats designed to clean the mouth and teeth such as Greenies.