I've had these spots appear on my stomach for the past couple of years. They seem to pop up immediately following my gym session. They do not itch, nor are they raised. As they heal the affected skin is lighter than my normal pigment.
Looks like Tinea versicolor which is caused by a fungus. Try washing the area with an anti dandruff shampoo, especially one with Selenium Sulphide as the active ingredient. Work into the area with a soft brush and leave on for 3 minutes . Then rinse well. Persevere with this treatment for 3 months and hopefully it will stop. If not you may need to see your doctor who may give you an anti fungal cream or oral medication.