Teething can cause bad breath which is common. If it's really, really terrible, you may want to take her in and make sure she doesn't have something else wrong.
We once had an older dog come in with terrible breath. The vet thought it was just poor dental hygiene and time for a teeth cleaning. When the dog was put under and the mouth opened, we discovered a small stick lodged in the roof of his mouth. There was a horrendous infection that was causing the foul odor :yikes: