What is this post-tummy tuck bulge?

Although there is still healing underway 6 months after a tummy tuck, major contour problems should not exist.  You picture is difficult to interpret.  The abdominal wall may have been poorly repaired or perhaps not addressed at all.  You might have had an untreated seroma that has become organized and hard.  I would press a good board certified surgeon for a specific answer.

At this point in time of your recovery, post tummy tuck surgery, swelling should have subsided and any complications may be corrected by your plastic surgeon. I recommend consulting witha  board certified plastic surgeon to discuss the best and safest possible option for your case. If you need revisionary surgery or any procedure to correct the complication or issue it will improve the bulge in that area. Unfortunately the photo you provided makes it difficult to diagnose the issue or cause of the bulge. It may be various issues such as, an acumulation of fluid, known as a seroma, a hematoma, irregularty in fat distribution, or muscle diastasis. Consult your plastic surgeon as soon as possible. Take care and good luck.