When using my shower or filling the bath, the water goes cold?

If the problem only occures at one location the problem is in the hot stem or debris in that port of a single handle faucet. Basic but specific plumbing tools may be required. So, consider a plumber. If not start by turning the water off to the faucet to disassemble the hot side. If it is Delta, remove the rubber seat and spring. If it is Moen remove the cartridge. Otherwise remove the stem. Hold a towel over the valve and bump a shot of water through the faucet body for a split second. This may clear the obstruction and reassemble the faucet.

John
July 2007