It could be a heat rash, if it only comes up when you get hot, does it go away when you cool down, or a couple of hours after you've got out of the sun or whatever?
Heat rash, also known as prickly heat, is like lots of little red bumps or blotches, it's winter and i can't remember exactly, and it feels like lot's of prickles on your skin (hence "prickly heat").
You should try wearing natural materials, like cotton, and loose clothing, use non-bio washing powder (not related to prickly heat but it's a common skin allergy) and keep cool and stay in the shade if possible.
Skin irritations are the worst, believe me, i have loads, especially when you consider that the epidermis is the largest organ in, or rather, out, the human body, so hope things work out for you, and remember, the worst thing you can do with skin irritations is scratch.