What could cause my showers hot water to not work well after water main work?
Sounds like there's gunk stuck in the shower line somewhere between the branch for the shower and the shower's hot water valve.  HOPEFULLY it's stuck right there at the valve.

One thing you can do is shut off the hot water in the house, then remove the knob and valve stem for the hot water in the shower, then turn the water back on.  Water will flow out of the valve (I'm assuming the valve is in a water proof area like the shower itself) - and hopefully this will flush the gunk out of the line, restoring the water flow.