I bought a pair of new denim capris the other day and I almost always wash my stuff before I wear it but well, laundry needs to be done

I wore them yesterday sitting out in the sun and when I went to take them off I had huge red and white splotches all over my thighs. They mostly faded after a couple of hours and are still there a little but WTF is that? I've never had anything like that happen before. It was almost like an uneven sunburn through my pants. I guess I'll try washing them and see if it happens again.
Formerly known as elmoali

Re: NPR: New jeans and a chemical reaction?
I used to work retail and opening a box of denim was always interesting due to the strong clohrine(sp?) smell, residual from the dying process. Even though you can't smell it when you bring them home from the store, many items have chemicals on them still. The sun plus any perspiration plus the chemicals probably gave you a slight chemical burn.
Always, always wash denim before you wear it. Also towels, they coat towels in something to make them feel softer when they're on the store shelves.