From your pictures, and the way that you describe the rash, I believe that you have a fungal infection. The type of fungal infection that I think you have is called Tinea Versicolor. This is caused by a fungus and can happen to anyone, especially those who sweat a lot such as when working out or when they are in the sun. This is not contagious but it can spread to other parts of the body and will react to the sun. The best way to get rid of this would be to buy some Selsum Blue shampoo or Nizoral shampoo. While showering, get your body moist, rub this over your entire body where you have the rash, and stand outside of the shower for 15 minutes to let the shampoo soak in. Do this 3 times a week for 2 weeks and the rash should start to go away. If it doesn't go away completely then you should see a doctor for a prescription antifungal medication. I have added some links below for more detailed information:
Tinea Versicolor : Article by Craig G Burkhart, MD, MPHTinea Versicolor with picturesIf this answer helps please hit the accept button. Thank you.
__________________
Quote: You guys are the Marine's doctors; There's no better in the business than a Navy Corpsman...." Lieutenant General Lewis B. "Chesty" Puller, U.S.M.C
If you find my answer helpful please honor my efforts and click the Green Accept button.