Sewer gas smell in shower only with hot water running

[Dmrlbert]

I am trying to determine why I am getting a sewer gas smell in my master bathroom. This is not happening in my kids bathroom when I run the shower. I only notice the smell in the master shower after about 2 minutes of hot water. When I run cold water only i do not get this smell. We had a tub removed about 3yrs ago and they had to reroute some plumbing. When I look in the pipes I see standing water in the p-trap. Please any help is welcome as I don't want my kids breathing this gas.

 

[hj]

It is probably magnesium sulfide cause be a reaction between the hot water and the water heater's anode rod. HOW the showers are constructed will determine whether you smell it or not.