My shower stopped producing hot water. It is just lukewarm. The water coming out of the faucet get hot but after pulling up the diverter valve the make the water come out of the shower head the temp changes from hot to lukewarm although I have the knob turned to full hot.

I have a single valve that controls hot, cold and flow.

Help!:)

How new is this diverter? Does it have anti scald on it? Did this just happen? Or a slow process. What is the make of the diverter?
Need more info please
Thank You