How do you know it is mastitis and not simply that her breasts are full? If you can help to empty them, the firmness will be gone. Put a kitten on each one, keep moving them from nipple to nipple until they soften. If the cat isn't running a fever, doesn't have red, hot breasts, and isn't acting ill, chances are she is just engorged rather than having mastitis. Just get the milk flowing, so it will not become infected.
You can buy antibiotics OTC from any farm supply store.
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I just realized you meant the kittens were gone. Just squeeze out a tiny bit of milk from each breast to soften it a bit. Don't completely empty them. If you just keep the heat and a bit of the firmness out (a few squirts of milk) she shouldn't develop an infection. She will dry up. Just don't completely empty the breasts. Take out just a few drops to keep her from getting too sore. Next time keep the kittens long enough for her to wean them herself slowly. That will keep her becoming engorged. This happens if you wean off babies too quickly and too early. If she is already running a fever, you might have to get some antibiotics for her. But simply keeping some of the pressure and engorgement off her breasts will help greatly.
