Thanks for your query.Based on the facts that you have posted it appears that you have what is called as Ring Worm Infection
Please take anti fungal medications like Fluconozole orally twice along with topical anti fungal ointment like Meconazole for local application over affected area.
Ensure to wash your inner ware in hot water mixed with antiseptic lotion like Dettol
This should get cured within a week.
Use loose inner wares.to avoid excessive sweating