Hi to bad IM not in Norman Oklahoma this month so I could fix it myself
This sounds like there maybe a partial blockage in the H/W side of your showerbody MOST lightly a broken washer.
If your handy shut off the hot water supply disassemble the H/W side of the showerbody look inside for an obstruction and try to flush it out using the C/W pressure stand back.
Good luck SylvanLMP