Herxing from shower?

Itching can be from a lot of things.

You mentioned products. Do you use only fragrance free products? If not, I would switch to those.

Is there mold in the bathroom? Some species of mold cause itchy skin.

Lyme can change your body, especially your immune system, and cause you to develop allergies and sensitivities to things you never noticed before.

Do you have dry skin? Hot showers can cause dry skin, which definitely will itch.

For relief, I would recommend dry brushing if you don’t already do this (unless you have painful sores). Then soak in a warm (not hot) epsom salt bath. If you’re not doing these baths regularly, start low and slow (1 cup for about 10 minutes; work up to 2-3 cups at 20 minutes). Adding a cup of baking soda to the water can help soothe skin. Then shower after the soak. Try standing in cold shower for a minute just before getting out. You can use organic unrefined coconut oil as a moisturizer afterwards.

Detoxing can cause skin itching and irritation. If you have a lot of die-off and an overload of toxins, it can affect your skin.

I second the mast cell idea. If you have histamine issues, your blood vessels are already dilated. Going into a hot shower dialates them further and lowers blood pressure, which could cause symptoms of POTS. Exercise is also known to release histamine, but I don't know why. I'd suggest taking a luke warm or cold shower and see if you get the same reaction.