Why do i get hives everytime i have a shower?

Hives from hot water: Hives when showering are called a "physical urticaria." these types of urticaria mean the histamine in your mast cells is for unknown reasons, easier to release than other people's histamine. Sometimes the stimulus is heat, sometimes cold, sometimes water, sometimes vibration. When the hives are small, it's often called cholinergic urticaria. Usually, antihistamines can block the symptoms.