According to Dr. Jaliman, a sun rash is an allergic reaction. "[It's] also known as sun poisoning or sun allergy and is an allergic reaction to exposure to the sun's ultraviolet rays," she said.
Dr. Nussbaum added that it may show up a little while after you've been in the sun. "It typically starts 30 minutes to a few hours after exposure to sunlight – and it may itch or burn."