Heat rash.

I sat outside in 66 degree Fahrenheit weather for about 45 minutes, just enjoying the sun. I came inside and I’m covered in a rash. This also happens when I shower in anything warmer than lukewarm water 🙄. Any tips? Is this histamine? Ugh it’s so annoying. I also get shaky after sun time. Yes, I am drinking water with sea salt in it.