Smell after upstairs shower in Downstairs bathroom????

after using the upstairs shower a GROSS smell comes out of the downstairs bathroom???

when someone takes a shower a strong "sewer/sulphur" smell starts coming out in the downstairs 1/2 bath- very strange- and occasionally the smell comes from the upstairs shower as well but not always- any ideas??? thanks!

You most very likely have a partially plugged vent pipe on the roof that is causing the sink traps to be siphoned when water runs through the drain system. Traps keep sewer gas from entering the home, unless it gets "drained" out by siphoning like this.
Go up on your roof (carefully), and clean out any leaves or other debris from the tops of the main vent pipe sticking through the roof that you can reach by hand, and then flush it down with a water hose sprayer.
Good Luck!
Mike