How hot is the shower?
I'd guess, and that's all it is, that any sudden change in temperature, up or down, can accelerate the HR, no idea by how much.
I'd guess my shower water is no more than 90 degrees, but "who knows"? I've never measure the water temperature or my HR in the shower...there must have been something to bring it to your attention, i.e., fast and heavy. I think I am pretty much unaware of my HR, even if over 120.