Short-term bad breath may be associated with tonsillitis, gastroenteritis, acid reflux,
diabetic ketoacidosis, or sinus infection. Chronic halitosis may be caused by poor oral hygiene, gingivitis (gum disease), oropharyngeal infections or a dental abscess.
I suggest you to get it examined from an ENT specialist. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided.