I have a problem with the smell of sewer gas smell coming after we take a shower or a bath, that is the only time it happens.
We have a drain in the basement that we plugged because it never gets used and there is water in the trap. Where could it be coming from and why?

Most bad smells in tubs, lavatorys and showers are caused by rotting hair matted with grease from the soap and from rotting food in kitchen disposals Try this for the smell. Tonight before bedtine Take a 1/2 gallon of bleach and pour it down the drain and let set over night. Next morning ,(and this is important) flush out the drain with two large pans of boiling water to loosen the grease and flush the mess out. The bleach will make the hair slippery and began to dissolve it and the hot water will melt the grease and flush the mess away.
This should make your drains smell better. With all chemicals used in drain cleaning proper ventalation should be observed. Good luck, Tom