It can be due to hyperpigmentation skin which is darkening of the skin that occurs when an excess of melanin, the brown pigment that produces normal skin color, forms deposits in the skin. For this you can go in for topical treatments like hydroquinone, azelaic acid, corticosteroids, tretinoin cream which are available under prescription or for laser treatment, Chemical peeling or microdermabrasion.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.