This seems to be a fungal infection. It is known as Intertrigo. You need to start on a course of anti fungals antibiotics to cure it. A course of Tab fluconazole should help. Also anti allergies will help you alleviate itching.
Next Steps
Keep the area as dry as possible. And change your inner wear at least twice a day
Health Tips
To prevent further recurrence keep using antifungal dusting powders.