You had a home inspection, right?
The home inspector should have checked to make sure the plumbing vents were clear and working properly.
Take the shower head off and soak it in a pan of vinegar...could be plugged up.
If your shower is draining properly and just stinks....it probably isn't plugged up.
When we had problems with our septic tank (4th or 5th guy out here was the one that figured out the pump was dead
)...when we showered in our bathroom, it backed up into the tub in our bath, the powder room toilet, the dishwasher and on of the kitchen sinks. Not in the laundry room which is also on the main floor or any of the 3 bathrooms upstairs.
We didn't have a smell though.
When we were in the worst of our drought the last couple of years....sometimes the water coming out of the faucets would smell really awful....is the water table where you are, really low?