What is a normal heart rate increase when taking a hot shower? it feels like my heart is racing abnormally fast and it sometimes causes a panic attack?

A hot shower will overheat your body causing your blood vessels to dilate to help cool off your body. The hot water causes a drop in blood pressure and a hemodynamic function occurs to counteract the drop in blood pressure by increasing the heart rate and pulse. Now you don't have to have a panic attack since you know why. Cool down the shower a little.

I do not know of a table that has been published on this but can speed up alot. Data from Japan is that regular hot baths lower risk of heart disease. No good data in US but worth doing I think.