I only have warm wter in the shower while the other sinks have hot water. The shower plumbing is about 6 years old and the problem just started recently. The faucet is a single handle preasure balance type.

One of two thing could have happened:

1. Your faucet has hot water balancing device inside. Depending on the faucet you have, it "slides" from side to side as water pressure changes. That "slider" device may be stuck. Since the faucet is 6 years old - I suggest that you: Turn off water / remove trim / remove cartridge / go to Home Depot with this cartridge in your hand / purchase EXACTLY the same cartridge / install it / install trim / turn water back ON

2. There is an anti-scald guard ring behind the handle. It could happen that it slowly wiggled out and re-positioned itself in different, less hot, position. Solution: Remove handle, remove anti-scald ring, reinstall handle.

Let us know how you did...