Thanks for the query.
You might have developed furuncle
These are deep bacterial infections.
Bikni line is prone to develop this.
As there is lot of friction and also hairs are present.
Another possibility is fungal infection
due to sweat.
Treatment is with antibiotics or antifungal depending upon the condition.
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.