Could be you might have come in contact with a poisonous plant like poison ivy, poison oak, sumac. An abrasion from a seat belt, back pack or something could cause some skin infection or folliculitis. Germs are already on the skin and then it gets some kind of point of entry. Ant bites, or mites might also lead to some infection. Shingles is more like clusters of cold sores, but it could be a virus.