Not uncommon. I recommend going to an ENT first. A Neilmed Sinus Rinse may help. If nothing else works, a Flip-Turn Sinus Flush will get rid of it if it's an infection. See section 17 for other causes of sinus stench.
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground.