Oh, oh, OH!

I thought of something else! You're working outside, right? Are you working with plants by any chance, and are there many weeds or wild plants around?
There is a small group of plants, of various types, that cause a condition called phytophotodermatitis. One is wild parsnip. If you brush against this plant and then expose your skin to sunlight, the area the plant touched reddens, looking and feeling like a sunburn. The extraordinary thing about this is that the redness can remain for months.
It would be pretty easy to walk into a plant like this.

But oh, that probably isn't very likely at all, considering the circumstances. I think I got a little carried away; I get so excited about plants.