Stephanie:
It is certainly possible to develop an allergy or sensitivity to the sun, even if it never happened before, and even if you use sunblock (since the rays which cause allergy are often not blocked by sunscreens.
I can't be sure, however, from your description whether that is what you have. If it happens over and over, that would be better evidence. What else could it be? Hard to say, since people sometimes develop various rashes which come, itch, and go, and we never do find out why.
Next time you'll be exposed, try using a broad-spectrum sunscreen with UVA protection (see the label) -- that may help.
Good luck.
Dr. R
I personally believe it is a vast right-wing conspiracy. But they may just have sensitive skin.
it's there any medication to make this go away?