Experiencing foul odor in my mouth that seems coming from deep inside instead of dental causes. what is likely the true cause?

Halitosis: Bad breath can be caused by a number of different reasons, including, acid reflux, diet, sinus infections, tonsil stones, cavities, and periodontal disease. Without an exam and a medical history, it is impossible to guess the TRUE cause over the internet. Please visit your PCP and dentist to rule out any of the above possibilities.

Many Possibilities: Most bad breath comes from bacteria on the far back of your tongue or throat. Your dentist can recommended a mouth-rinse and cleaning aids to help with that. If it's coming from "deep inside," that usually means from the stomach. Gastric reflux/GERD are the most common. This would have to be diagnosed/treated by a gastroenterologist or your primary physician. Sinus problems are also common causes.