Hello,
One of the possibilities for such discoloration can be tinea versicolor. The other cause can be hyperpigmentation skin which occurs when an excess of melanin, the brown pigment that produces normal skin color, forms deposits in the skin.
Tinea versicolor causes the affected skin to change color and become either lighter or darker than surrounding skin.
I suggest you to get it examined from a dermatologist. 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.