Thanks for your query
It could be -
1. an allergic reaction
to something that touched your skin,
2. an irritant (non-allergic) reaction, or
3. a fungal infection
4. any number of other things.
A dermatologist needs to see it in person to narrow down the possibilities.
So i advise you to consult a dermatologist
to locally examine your lesion and diagnose it. Then you should apply locally as per his advice. Otherwise you may try our premium forum by posting the photo of your lesion so that we can diagnose it and advise you properly.