Odor from shower drain
A very likely cause, and one that is not too difficult to fix, is a partial obstruction of that drain (or even another drain) downstream or a partial or full obstruction of a vent pipe.

Are there any slow drains in the house? If so, snake them out. Go on the roof and inspect your vent pipe(s) for any debris, animal nest, ice plugs, etc. Run a large snake cable down the shower drain itself too.