A couple red flat lesions in chest and abdomen area come n go when i sweat becomes darker in color ( red) what can it be?

Rash: Could be allergies, you can develop allergies ( contact/environmental/ food) even if you never had them. They darken when you sweat because you are vasodilating/ increasing blood flow to the area when sweating. Would test for lyme disease. It could also be related to change in vasomotor tone to triggers/stress/catecholamines/histamine, rarely autoimmune conditions. Follow up with a doctor