Several things can cause a rash after sun exposure. Polymorphic light eruption is a common itchy, bumpy or blistering rash that appears on sun-exposed skin, often in spring or early summer, and tends to settle over days. Prickly heat (heat rash) causes small itchy spots when sweat ducts are blocked in hot, humid conditions.
Sometimes the reaction is between sunlight and a substance on or in the body, called a photosensitivity reaction. Certain medications, fragrances and plants can make the skin react strongly to sun, producing exaggerated sunburn or a rash where light hits the skin. Some autoimmune conditions can also cause light-sensitive rashes.
Protect your skin with shade, clothing and broad-spectrum sunscreen, and check with a pharmacist or doctor whether any of your medicines increase sun sensitivity. See a doctor if the rash is severe, blistering, widespread, recurrent, or accompanied by feeling unwell, as it may need specific treatment. This is general information, not a diagnosis.