It sounds like you are experiencing what is called post-Inflammatory hyperpigmentation which can occur in individuals after their skin has become inflamed, such as from a rash, acne lesions, post peel, lasers etc. I would recommend you make an appointment with a board certified dermatologist to examine your rash and determine its cause and treatment. The first priority would be to clear the rash up and then your dermatologist can also recommend treatment for the post-inflammatory hyperpigmentation you are experiencing.