It could be because of many things, Acne , Folliculitis ,Furunculosis, H Suppuritiva, Contact Dermatitis to soaps, synthetic under garments..it should be properly analysed
Next Steps
Keep area clean and dry.. avoid synthetic tight undergarments, tight pants, loose weight if you're having fatty bums , contact me on Practo pro for further medical advice
Health Tips
Use mild soaps , avoid prolonged sitting, avoid sweaty surrounding, apply powders and take medical advice