I have constant bad smell in mouth..why is it so?

Esophageal diverticu: You may have an esophageal diverticuli. Some times food get trapped in this an it smells. Make sure you are brushing your teeth and use mouth wash. If this does not solve it then see a doctor to rule out a diverticula.

Halitosis: Need for thorough evaluation for cause-many possibilities-side effect of medications, improper brushing regimen, acid reflux, etc. Too many possibilities so a comprehensive evaluation by dentist is needed.