A buried stitch may be the most likely culprit.

The belly button usually heals within 2-3 weeks after an abdominoplasty. depending on the type of sutures used, an infected suture, a "spitting suture" or a suture that should have come out but is still in may be the answer.

A quick visit back to your surgeon should easily clarify the source of the bad smell. If it is a retained suture, removing it is a simple process. If antibiotics are needed your surgeon will prescribe them.

The value of washing wounds with peroxide is overstated. Peroxide will delay healing. Mild soap and water is all you need.