Upstairs bathroom... double cabinet... double sink. Only the attic is above this bathroom. No order in any other bathroom or any where else in the house.

In our upstairs bathroom there is a strong septic smell only after I've had a long shower. There is a slight odor underneath one cabinet only... the other cabinet smells perfectly fine. We had a plumber in and found out our toilet was properly sealed so that was fixed. That fixed a part of the problem. The smell under one sink continues. When the water is not running the smell goes away. If you run the water a little bit there is a little bit of an odor but not much. A long shower will produce a pretty icky smell. No foul air around tub or shower or toilet... only underneath that one cabinet!

Any suggestions?? Advise?