Google "sun allergy" and you'll get the explanation. It's actually photo-sensitivity. It's hard to figure out because I get the rash near my joints and not necessarily where I got the sunburn or over exposure. It has helped to limit exposure by staying under the umbrella and using a higher SPF. For the longest time, I thought I had a shellfish allergy. Not much you can do once you get the rash except use an anti-histamine topical cream and take Benadryl.