Welcome to HCM.
Most likely it is due to infection,inflammation of that area might be followed by improper hygiene after sex,scratching after perspiration,tight undergarment.
Apply antibacterial, anti fungal cream on it.
Oral antibiotic like oflox
will be helpful.
Take proper local care,use medicated soap to clean,don't wear tight undergarment.
Consult your physician for better medical management.