1. DIY – poor 2 cups of Clorox into the shower.
2. DIY or plumber – Run a roto rooter machine in the shower drain
3. Plumber – New Shower pan

First you could try pouring Clorox into the shower, if that resolves your problem great.  If the smell comes back you could try cleaning your drain with a machine roto rooter machine, if that resolves your problem great.  If the smell comes back again or that step did not work at all then you may have a faulty shower pan.  The solution is to tear out the shower and install a new pan.  We would be glad to help with step two and three if necessary.  We have many calls from frustrated customers who have water pouring into their closets because a shower pan was not properly installed. Save time and money by using a licensed plumber the first time.