Horrible sun or sunscreen reaction! help me figure out which?

Try test patch: I would try a brief test patch of the product along a hidden body area or for a more precise identification see an allergist or dermatologist for scratch testing or possible serologic evaluation.

Simple: Put the sunscreen at full strength on the pad of a Band-Aid. Then place this on an area of your skin that is not broken out. Check if every few hours to see if anything is happening. If turning red, it is a skin reaction to the sunscreen. If not, leave it on overnight to be sure. If no reaction within 18-24 hrs it is not the sunscreen by itself.