You may have some hair partially clogging the drain. It holds enough moisture and soap scum to get stinky. Try using a "snake" in the drain to make sure it's clear. After that, a good dose of hot vinegar will help remove more soap scum. The drain pipes can also get a build-up of soap scum that can start smelling. Call any "good" plumber for more information.

-Grainlady

I agree with Grainlady,

You probably have hair in the drain. Suggest you go to Lowe's or Home Depot and get this inexpensive drain cleaning tool.

Here is a link that might be useful: Drain cleaning tool

Grainlady is right. I could not figure out the smell in my bathroom and finally dug in that drain and almost died with the glob of hair and gunk I pulled out. Ugh! DD has very long hair and there was a head full in there. Now, I pour down drain cleaner once a month without fail.