Assuming that you're brushing and flossing
regularly, bad breath
can be a sign of a chronic sinus infection or an infection in your mouth and/or throat.
If you are having any pain in your forehead or cheeks or tenderness in your mouth or throat and/or a stiff neck, see your doc right away. Post nasal drip
and gastrointestinal problems can also cause bad breath. It's worth it to have an evaluation.
Answer assisted by David Miller, Medical Student