since summer is over i've started using the hot water again and now it has low water pressure, low enough to not being able to start the gas water heater. oddly, the sink faucet of the same bathroom has good pressure and can easily have hot water. i've notice that we have very hard water, but i replaced the shower head and the problem persists. i live in a three year old one store house with two bathrooms.
Since you do have hot water coming out of other faucets it is most likely your shower valve.
Many of the single handle shower faucets have a cartridge valve that is very easy to replace. The cartridges are specific to the faucet so you will need to look up your model. I had a similar problem with mine. The cartridge cost around $60 and took about 10 minutes to replace.