Hi,
This could be sweat dermatitis most probably complicated by friction, eczema or bacterial skin infection or heat rash.
Try to avoid sweat accumulation at the sites and use a powder. Keep the area clean and wear lose, comfortable clothes.
Since when have you noticed the lesion?
You could take oral antihistamine medications for the itching and apply calamine lotion at the site.
It would be best to consult a skin specialist as nothing can be said with surety without having a look at the lesions.
Let us know about what the doctor advises and post us if you have any other doubts.
Regards.
It sounds like herpes to me. You should get tested immediately.