Ok , then again it is probably not a sewer gas prob. As said before take the shower head apart and make sure there isnt mold growing .
The problem may be because other traps in the house may be dry. Run water in every fixture in the house for 20 or 30 seconds ..
I have a notion that I hope is wrong but after doing the above run the shower for a while and see if it stinks again and let me know.