could be sinuses or gerd(gastroesophageal reflux)
Watch your diet. What you eat affects the way your breath smells, so avoid foods like onions and garlic, which contain sulfur compounds that give them and your breath a strong odor. Cheese, orange juice, and soda (including diet soda) can also cause bad breath.
Brush and floss. Brush after eating and before going to bed, and floss once a day to remove food particles that can decompose and attract bacteria.
Drink water. Even if you don't eat for hours, at least drink water to help wash away food particles and bacteria. In fact, people get "morning breath" because there's less saliva in their mouths during the night to wash away bacteria.
Some people swear by MintAssure, an all-natural breath pill that contains parsley seed oil and natural peppermint flavoring.
Get regular dental checkups and professional cleanings help prevent tooth decay and gum infection two conditions that can cause bad odors in your mouth.
If you still have bad breath, talk to your doctor. A medical condition, such as a sinus or lung infection, postnasal drip, or diabetes, may be the blame.
Could be the food you are eating, maybe try a mouth wash ,Did the dentist tell you your teeth were okay and no gum disease?I know there is something called halitosis maybe have a word with the doctor