I have hot water in my bathroom sink but not my shower?

I got a new water heater professionally set up only a few months ago, all new parts. I went in the other day and the shower had no hot water at all. It always had low pressure but never none. The sink had full pressure, very hot water. I fiddled with the knobs that turn water on and off to objects inside the house (the hot water pipe one, not the cold one) and it turned off the sink hot water completely and gave the shower lukewarm, semi-pressurized water. Turning it back, the sink worked again and shower didn't. Any ideas how to get them both working again with full pressure and hot water?