Hello,
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.
2.
melanoma, 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.
http://www.healthcaremagic.com/doctors/dr-rahul-kumar/64818
Wishing you a good health.
Thank you