Hi thanks for writing here.It seems you are suffering from intertrigo of perianal region.It usually occurs in flexural areas ,where there is friction of skin leading to maceration follwed by secondary infection with fungus and bacteria.Keep area dry, wash with antibacterial liquid soap after toilet,start using antifungal powder like candid at day time ,apply antifungal and steroid combination like candid b at night,take antihistaminincs like tab.cetrizine at night,use dusting powder constantly even if you do not have itching,which will prevent recurrence,,use loose under garments,avoid constant sitting at one place,rule out diabetes also fissures and piles which will produce soaking and will lead to irritation.You can consult a dermatologist for this.Take care.