a couple days ago this started. no matter what position I turn the handle towards the left it's always very hot. and the same if I turn it to the right. very cold. I'm assuming there must be some sort of regulator. this doesn't happen with any other of my taps. nor does it happen when the bath is run.
Your shower mixing valve probably has an anti-scald feature with either pressure balancing or thermostatic control. These can get out of adjustment.