Every time I go in a sauna and come out I smell like I had pooped in my swim trunks. What could be causing this?

Not sure but. Itcould be from the germs or bacteria in the skin that come out of the skin pores after exposing yourself to the heat in the sauna. Take care! http://www.Mayoclinic.Com/health/ovarian-cysts/ds00129.
Chemicals . You are probably smelling the results of the chlorine (or bromine) that is added to the sauna to keep the bacterial count down. These compounds can release gases that are somewhat noxious.