I have noticed that my pressure for the bath tub is very low. Its only low with the hot water. The cold water's pressure is fine. What could possibly be the problem?

One handled valve? What's the brand name? Sounds like the hot water iinlet ports on the cartridge might be clogged. Shut the water off to the valve and open it up. Remove the cartridge and check and clean the inlet ports. Before you button up turn the water back on to flush out the supplies. Good luck, tom

... also, in addition to Tom's comment... if it is single handle faucet the you can also have anti-scald ring that needs to be adjusted. Please, answer Tom's questions so we can go into details...