The answer to 'why is my shower foul smell' is that a foul odor in the shower is often caused by a buildup of organic matter, such as hair, soap scum, and bacteria, in the drain or P-trap. This can create an unpleasant, sewage-like smell. Other common causes include a malfunctioning ventilation system, a clogged drain, or a dry P-trap that allows sewer gases to back up into the shower.
If the smell is coming from the drain, you may be able to resolve the issue by using a plunger or drain snake to clear the clog. You can also try pouring a baking soda and vinegar solution down the drain, followed by hot water, to help break down the buildup. However, if the problem persists or the smell is particularly strong, it's best to call a professional plumber to inspect the issue and provide a more thorough solution.