The most common cause is the most obvious: heat rash, also known as milaria. Heat rash typically occurs when sweat ducts become plugged, which can cause tiny water bumps that may resemble pimples. "It will usually show up on the upper body (head, neck, chest) or where skin rubs together, like between the legs or in the armpits," David Lortscher, MD, a board-certified dermatologist, told POPSUGAR. Although it's unpleasant, heat rash is typically not dangerous and can be treated at home with aloe vera, cool compresses, and hydrocortisone cream. If you develop heat rash, Dr. Lortscher recommends staying cool and dry for the next few days, which will allow the rash to heal. If it doesn't resolve itself, it's time to call your doctor.
But milaria isn't the only reason you may have a rash after spending time in the sun. Certain medications can increase photosensitivity and put you at a greater risk for both sunburns and skin rashes. Dr. Lortscher explained that doxycycline, an antibiotic that's frequently prescribed to treat acne and Lyme disease, "can cause unexpected sunburns or blistery rashes on sun-exposed skin." Although it doesn't increase the likelihood of sunburn or skin rash for most people, he recommends taking precautions if you're on this antibiotic — be vigilant about wearing sun protection and pay attention to how your skin reacts to the sun. Likewise, if you use retinoids like retinol and tretinoin, take them at night and diligently apply sunscreen during the day. "Retinoids make the skin sensitive to the sun, causing or exacerbating redness, peeling, and irritation," Dr. Lortscher said. "UV light degrades tretinoin and can also make you more sensitive to the sun."
Heidi B. Prather, MD, FAAD, a board-certified dermatologist at Westlake Dermatology, told POPSUGAR that heat rash can also be caused by ingredients in perfume or fragrant lotions — so if you develop a rash once after wearing a certain product, ditch it the next time you're in the sun because it could easily eliminate the problem. She also noted that sun sensitivity and heat rash are symptoms of certain illnesses. "A number of medical conditions are associated with sun sensitivity, such as lupus and rosacea," Dr. Prather explained. "Porphyria is more rare and can result in blisters on the skin after sun exposure." If you consistently have a strong reaction to the sun, it's certainly worth having a discussion with your doctor to explore the possibility of an underlying condition.