Shower cold but other water hot?

I have a wall mounted shower that is approx 1 year old (sorry no idea of make can't see a label on it) it has a thermostat on one side and it has always been set at 38. Lately I have had to turn this up and up just to keep it warm, now it is on full and barely warm enough to stand in, but bath and taps have very hot water. Feeling the pipe going into the shower these feel hot. HELP don't want to have to call out a plumbe this close to Xmas