Thanks for the query,
There are two possibilities,
1. You might have developed fungal infection
It is called as tinea nigra
Your description matches with this.
Please get a KOH mount done,
This can confirm the fungal etiology.
Treatment can be with oral antifungals as well as with topical antifungals.
, which is a cancerous condition,
A biopsy may be needed to differentiate between them.
Please meet a dermatologist
for exact diagnosis.
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.