The most likely cause is that the water in the trap under the shower is being sucked out - assuming you have a trap. If you can look into the trap see if there is sufficient water in it to provide a seal. If not you will need to fit an air admittance valve in the pipe from the shower.
hi, like many others on here, i have one shower which is making the most horrible sewage smell. i held a lighter near the plug and found that the smell comes with enough force to blow out the flame. the shower is on the second floor, any ideas what the problem could be?