Plumber connected shower and toilet drain weirdly, now toilet backs up into shower
From your description it doesn't sound like you are actually seeing sewer waste water emitting from the shower drain, but you do smell sewer gas in the shower area.  Most likely you have a drain ventilation problem.  When the toilet is flushed, the water entering the drain displaces the air that's already in the drain pipe.  If the pipe doesn't have sufficient ventilation it will push the stinky air out of the easiest opening  (your shower drain).