You could try an elimination diet where you avoid all the common allergens for a week or two and see if it improves. The most common food allergies are wheat/gluten, dairy, soy, and nuts.
One way to do it is to just try to eat very simply. Avoid anything packaged. Eat lots of fruits, vegetables that you purchase raw and cook yourself. Potatoes are good for your starch, or rice. Eggs for breakfast are safe. If you need cereal or other packed foods, make sure you buy gluten-free.
Do this for a week or two and see if the rashes improve. If they don't, then it's probably not a food allergy, at least not to the most common foods. But some people have different ones, like citrus fruits.
It could also be a fungus. You could try an over-the-counter antifungal on one small area and see if you get relief.
Good luck.
It could be an allergic reaction to the UVA or UVB rays, however, it could be something else such as poison ivy, poison oak, poison sumack, chiggers, etc. Have you been bit by a tick lately? Do the areas hurt along with itching? If you say yes to either of these, then it could be lyme diseas, rocky mountain spotted tick fever or even shingles. it could also be a fungus of some type. You should really see your doctor since he/she is the only one who can properly diagnose you.