Thanks for the query,
There are three possibilities.
1.Lichen pigmentosus, an autoimmune disorder
, due to low platelet.
, a cancer.
will be required to reach a diagnosis.
Please meet a dermatologist
for final diagnosis and treatment
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.