What could cause low hot water pressure in the shower only?
I would first check the valve's anti-scalding setting.  You have to take off trim and this is usually a setting on the valve outer stem.  Make sure this is set to as hot as possible.  This could have slipped.  Next I would check the filters inside your valve cartridge.  There is usually a filter for cold and another for hot.  The hot filter could be clogged and need cleaning (very seldom do these need replacing except the rings).

Lastly you might need a new cartridge or to have yours rebuilt.  If you know the model this might actually be easier than all of the other troubleshooting but often isn't needed.   The cartridges can cost as little as $10 and up to a couple hundred.