Hello and welcome to health care magic. Your description is typically of a fungal infection possibly tinea. The area where you have the irritation is one of the commonest areas were fungal infections are found. It s easy to control and kill them but it s pretty difficult to prevent recurrent infections in future. If you visit a Skin specialist he will prescribe Clotrimazole ointment which you can apply in the morning and at bed time. Wash the area with warm water and soap and dry the area before applying the ointment. This will get rid of the infection in 2 to 3 days. But the problem is the fungal spores which will be present in all your undergarments and towels or whatever clothes that comes into contact with it. Those spores can cause reinfections 2 to 3 weeks later. I usually suggest my patients to get rid of their old undergarments and buy new ones. If you still want to use old ones, then boil them in water for 15 minutes, put them in sun and later iron them especially in the folds. Same can be done with towels as well. Hope this discussion will be helpful to you. Thank you.