exposure. You may be experiencing a photosensitivity reaction. Photosensitivity reactions involve a whole spectrum of skin conditions. Solar urticaria may present with a rash few minutes after exposure to the sun. Chemical photosensitivity may present with skin rash after application of creams and lotions coupled with sun exposure.
Which one applies in your case? Is your skin condition aggravated by certain lotions and creams applied on the skin?
A sweat rash or miliaria is also a differential in your case. This however appears in covered areas of the body. A rash is usually present.
I only use sunblock which seems to lessen the severity of the burn. Otherwise, I don't use perfumes or lotions or i'll start itching. I don't really sweat a lot though. Only when I work out really hard.