What Causes Bad Odour While Sneezing?

I'm a 51 yr old male, 6'1", 225#. In very good physical condition. I have my teeth cleaned twice yearly, brush, floss and rinse at least 3 times daily. I never eat onions or garlic. I have never used tobacco and I've never drank coffee. I can't smell myself until I sneeze and then I smell a musty and nasty odor. I had an uncle who had an odor at an early age who was told he had "catarrh" and was told to smoke--he died at 61--lung cancer. Noone tells me I smell, but I've been offered gum by strangers--It keeps me from living a normal life. Immediately after brushing drainage in my throat takes away the freshness of brushing. please help!!!