Why is there a foul odor in your basement after showering?

Answer 1

Most likely answer is due to a damp problem, get the shower waste and piping replaced to the main stack, re-waterproof the shower completely and tile over it mixing the grout with bond it (waterproofing additive)

Answer 2

There is no water in a p-trap somewhere. The p-trap is a seal of water that prevents sewer odors from coming up every drain in your house. When they are dried out of water, the gases from the sewer or septic can come back into the house. You can fill them up with water again or, this is an old trick for seldom used traps, pour enough vegetable oil into the trap to seal the p-trap. The oil does not evaporate!