I had DIEP flap surgery a year ago and a bulge along my incision line that swells

Occasionally, the nerves that go to the abdominal muscles are cut during the DIEP, causing a laxity of the abdominal wall.  This can lead to a laxity, mimicking a hernia.  The CT shows an intact abdominal wall without a hernia, and the bulge is only apparent after standing.I have had this occurrence in about 4% of my DIEP flap patients.  Plicating, or tightening the abdominal wall has been successful in improving this issue

Do you know exactly what was done in your surgery? Sometimes plastic surgeons do a "DIEP" flap reconstruction but do take muscle and fascia. They close the abdomen using mesh. On a CT scan you technically don't have a hernia but still have abdominal wall laxity which makes you look "pregnant". I would see your PS or even get a second opinion. 

Chances are you have a defect which is not detectable on CT scan. Anatomically the rectus muscle is preserved, however there is trauma to the lining of the muscle(fascia) and the surrounding tissues which may predispose anyone having this procedure to such defectsThe repair involves the use of mesh and a modification of a procedure called a components seperation. Your surgeon can discuss this repair more thoroughly with you or possibly refer you a surgeon with more experience with this procedure.