Todder...rash after sun exposure...what is it?

I get rashes after sun exposure, too (I'm a 27 year old female). They're not bumpy really, but they are red and splotchy. I can even get them after taking a really hot shower. My doctor diagnosed me as over-sensitive to the sun/heat. The rashes don't hurt at all, and when I know I'll be outside for longer than about 10 minutes, I wear 50+ SPF broad spectrum sunblock and re-apply it every 3 or 4 hours, more often if swimming or something. There's also an ingredient in some sunblocks that I've gotten a reaction from, but for the life of me I can't remember what it's called right now, sorry. Good luck to you and your little boy.