After using this shower valve over a period, I'm only now getting cold water from both the hot and cold side of faucet What would be causing this?

Does the shower begin hot?

How long is a short period?

Have you tried adjusting the thermostat on the water heater?

Please tell us the brand and the model number of your shower valve, it sounds like you have blockage on the hot side what you need to shut shut the water to shower or the house then remove the cartridge turn on the water to flush out the debris, Thanks.

John