The reason you feel weak after a shower is due to a drop in blood pressure.
When you take a hot shower your blood pressure drops some and can drop more severely temporarily in some people.
A combination of venous pooling in your lower limbs along with additional flushing where your blood pools in your skin as a response to the hot environment where the blood is diverted to the skin as a means to help cool off the body leads to a drop in blood pressure and can cause you to feel weak and even faint.
It's usually nothing to worry about and as your body cools off your blood pressure should return to normal.
After your hot or warm shower or bath, your warm body goes into a cool-down mode.
When that happens it then triggers your circadian rhythms that tell your body it's time to sleep.
So some people may feel tired or weak after a shower or bath.
If the water's too hot in shower or a bath or hot tub, your blood pressure may dip too low, which can make you feel dizzy or lightheaded.
By low blood pressure it means a systolic pressure (the first number in a reading) around 110 mm Hg or lower.
"A water temperature of 100 F° to 105° F is reasonable temperature.