Hello and thanks for the query.
Your symptoms point towards two conditions which should be explored.
These are 
Melasma/chloasma
Acanthiosis nigricans

Melasma/chloasma is a patchy brown or dark brown skin discoloration that usually occurs on face and may result from hormonal changes, as in pregnancy and during the administration of estrogen containing oral contraceptives.
  Chloasma usually affects women but occasionally is seen in young men who use after-shave lotions, scented soaps, and other toiletries.

Acanthosis nigricans is a skin disorder in which there is darker, thick, velvety skin in body folds and creases.
Acanthosis nigricans can affect otherwise healthy people, or it can be related to medical problems. Some cases are genetically inherited. The condition is most commonly seen among people of African descent, in part because it is easier to see in darker skin.

Obesity can lead to acanthosis nigricans, as can some endocrine disorders. It is often found in people with obesity-related diabetes.
Sometimes people with lymphoma or cancers of the gastrointestinal or genitourinary tracts can also develop severe cases of acanthosis nigricans.
You must consult a dermatologist and get the required investigations done to get to the diagnosis.
I hope to have answered your query however you may revert to me for any other query.
Thanks and best of luck.

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