Just taking a stab at the dark here but is it possible that it is the water coming out of the showerhead that smells? At my brother's house, he gets all his water from a well and if they don't keep the filter maintained properly, the water smells of rotten eggs. Or, if the workers below disturbed your water lines significantly or shut the water off for awhile, it might have resulted in old debris coming out of the lines, which could be trapped in your showerhead.
The pros should weigh-in on this but your p-trap provides a constant water seal -- it's designed to keep odors out. If there's nothing between your drain cover and the water at the bottom of your trap that could be causing odor, it seems to me that it must be related to what is coming out of your showerhead.