When the mucus (in case you have sinus infection)runs down your throat it reacts with the bacteria there and cause bad breath called as halitosis. It can be triggered by colds, flu, allergies, pregnancy, sinus infection, changing weather, deviated septum, certain foods, and environmental irritants (smoke, chemicals, pollution, perfumes, cleaning products etc).apart from this there are many conditions in which bad breath is seen like-gingivitis,toothdecay,enlargedadenoids,pharyngitis,bronchiectasis,alcoholism,foreign body in nose,kidnet failure,diabetes etc.you need complete physical examination and blood work,urine work and based on the result some appropriate investigation to diagnose the underlying condition and treat accordingly.so you better consult a qualified physician and get treated.