A foul odor coming from the shower floor indicates a clog.
Often the odor may be the result of bacteria growing underneath the metal plate that covers the shower drain. This is called bio-film and can be washed away.
Remove the drain strainer. With the cover off, you should be able to see into the drainpipe.
Cleaning can be accomplished by using soap and water and a long brush. If you thoroughly clean out the drainpipe and continue to smell the foul odor, don’t use a chemical drain cleaner, as they are ineffective in treating the problem. Liquid drain cleaners usually don’t make it past the clog and just matters worse.
If you can’t resolve the problem by cleaning the drainpipe, a call to a professional plumber will be necessary.