The most common reason for a rash in this area, especially if there is any irritation or itching is yest of fungus. Yeast likes a warm dark place to grow. I would tray an over the counter yeast medication like monistat, and apply to the area for a couple of days and see if that resolves it. the other possibility is a skin reaction to certain metals, like buttons or belly rings. Finding the offending item and limiting exposure and using an over the counter hydrocortisone cream will help.