Cesarean section is a routine operation, and abdominal incision is removed 7-9 days after operation, with good healing and no complications. Your case is considered as an incision-related complication, which may be scar hyperplasia, subcutaneous hematoma, incisional hernia, lipoma or myometrium rejection, lipoma, etc. Considering that it is related to menstrual cycle, further examination is needed to rule out endometriosis. I suggest you go to gynecological surgery for a follow-up visit. The examination is simple, and the cause and condition can be known through inquiry, palpation and ultrasonic examination. Don't repeatedly stimulate the tumor with your hands, so as not to expand the bleeding.

I hope it helps you. I wish you a speedy recovery!