Hi, A small firm lump between the vaginal wall and rectum could have various causes. Some possible causes may include: * Bartholin s cyst: This occurs when the Bartholin s gland, which is located near the vaginal opening, becomes blocked, and a fluid-filled cyst forms. * Perineal hernia: This is a rare condition where there is a weakness or defect in the muscles and tissues of the pelvic floor, leading to the protrusion of organs or tissue into the perineal area. * Abscess: An abscess is a localized collection of pus that can occur due to infection. It may require medical intervention such as drainage and antibiotics. * Lipoma: A lipoma is a benign fatty tumor that can develop in various parts of the body, including the perineal area. * Epidermal inclusion cyst: This type of cyst forms when the outer layer of skin cells becomes trapped beneath the skin surface, causing a lump to develop. Your sister needs to consult with a healthcare professional, such as a gynecologist or colorectal surgeon for any necessary tests or imaging and an accurate diagnosis. Take care. Hope I have answered your question. If you require a personalized response, kindly use the Ask A Doctor service and post a follow-up query. Regards, Dr. Priyanka, General & Family Physician