Bulge could be related to fluid, scar, or tissue injury

Fullness above your abdominoplasty scar could have a number of causes. These would include tethering of the scar to the underlying muscle causing buckling of the skin above.  In addition, it is possible you developed a small fluid collection under the skin which has not resolved. The firmness under your belly button could be related to fluid, or possibly some injured fat in the skin below which could also appear as a bulge.  It is also possible there is simply some additional redundant skin not completely removed in surgery. Followup with your plastic surgeon will help you better elucidate the cause of this "pooch".