You could have picked up a bacteria from the sewage and have it start growing in the infected mucus you already had in your sinuses. Or the smell could simply be a result of bacteria already in your sinuses from your sinus infection. I've had the same problem from sinus infections, without having done any emergency plumbing.
The easiest way to get rid of it is to go to Walgreen's or any chain drug store and get a Neilmed Sinus Rinse and irrigate out the infected mucus. That will work if it's only in your lower sinuses. If the infection has spread to the upper sinuses, then a Flip-Turn Sinus flush will reach it
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.
Of course, you can also go to a doctor or an ENT and try the antibiotic route. That will probablty work also, and is more conventional, but it will take more time.