A CT scan will help evaluate the nature of the issue with your abdomen. Many instance of lower abdominal swelling after reconstruction are due to swelling caused by limited lymphatic drainage due to the very long horizontal scar on the lower abdomen. I am sorry that you may also have issue with areas of fat necrosis in the breast reconstruction. These are harder to comment on but if there is swelling in the remaining mastectomy skin over the flaps then this contributes to feelings of firmness. A course of lymphatic massage can be done simply by requesting that your surgeon give you an order for physical therapy that is generic. You can find a physical therapist who is trained in lymphatic massage by searching online and then making some calls. I send each of my patients through a physical protocol designed for patients who have had the DIEP flap procedure. If there is residual breast reconstruction sweeping or lower abdominal swelling they are referred additionally for evaluation and treatment by a therapist who performs lymphatic massage.
You likely have some weakness because the muscle may not be a strong as it was before surgery. You likely need some sort of mesh support.
you will need possible mesh to support the abdominal muscle and fascia. Please see your surgeon or get a second opinion.