I just had a new bathroom faucet installed about a week ago. Today when I turned on the hot water side, I had full pressure, then pressure went low and has stayed that way. The cold side is fine. Did something break or could it be installed wrong? Thanks.

" Did something break or could it be installed wrong?"
Neither one! Most likely some minerals built up in the hot water pipe walls was disturbed and found its way into the hot water inlet ports of the cartridge. Shut the water off to the valve and pull the cartridge. Check and clear the hot water inlet ports. Now turn the water back on for minute to flush out the hot water supply and valve body. Reassemble and test. Better now? Good luck, Tom

Just wanted to let you know that you were exactly right! However when I went to shut off the hot water at the shut off valve underneath the sink, it wouldn't shut off the water. I had to shut off the main valve. Then when the faucet was taken off, there was a piece of hard rubber underneath the faucet cartridge. The shut off valve needed replaced, also. When we looked inside, a piece of the rubber lining inside the valve was broken off. This was the hard piece of rubber that was in the faucet cartridge! It must have went straight up the line to the faucet. Thank you very much!