The offensive agent in your sunscreen is likely PABA, and if you are sensitive to it you will get a reaction like that. Look for sunscreen that specifically says NO PABA. I use Coppertone Sport with no problems, but samples of a Baby Magic product produced welts and blisters the likes of which I've never seen. Read the label - yep, it was all PABA.

Takes a couple days for the rash to go away, but it does. Unfortunatley it looks pretty bad, but at least for me it was not painful, perhaps a little itchy...