Welcome to HCM.
It could be due to infection in the passage of perspiration gland.
It may occur after sex,perspiration,scratching and improper hygiene.
Apply local anti fungal,antibacterial cream.
If it is recurrent than oral antibiotic like oflox/ornidazole
,flucanazole-150 will be helpful.
Keep proper hygiene after sex,use medicated soap.
Get examined by your doctor and start treatment accordingly.