Swelling after first ab workout, 12 weeks post Tummy Tuck, Hernia Repair surgery. Is this normal and how can I avoid it?

As always, it is best to be seen in person ( by your plastic surgeon) for precise diagnosis and treatment. This is especially true in your case given the change in your physical examination after vigorous activity.
 Generally speaking, abdominal wall "swelling" after tummy tuck may be related to:
1. Swelling in the soft tissues.  This may take several months to resolve and may worsen with increased activity  or at the end of the day.   Patience  is required to allow for resolution of the swelling. The swelling occurs because of the interruption of venous and lymphatic channels that occurs during the tummy tuck operation.
2. Fluid accumulation in the space between the skin and the abdominal wall muscle.  this may consist of blood ( hematoma)  or serum (seroma).  This fluid accumulation can generally be diagnosed by physical examination ( occasionally ultrasound  may be helpful).  Treatment consists of aspiration;  several episodes of aspiration may be necessary. 
3. Separation of the abdominal wall muscle repair may lead to a swelling/bulge appearance. This may be diagnosed on physical examination  with your surgeon examining you in different bodily positions. One of the steps of a tummy tuck procedure involves reapproximation (plication)  of the rectus muscles.  These muscles have spread apart during pregnancy and/or weight gain. Bringing them together again in the midline helps to “tighten” the abdominal wall as well as to narrow the waistline.
4. Residual adipose tissue may be confused for swelling. Again this is most easily diagnosed by physical examination. Additional liposuction surgery maybe necessary to improve the results of surgery.
Generally, it takes many months for swelling to resolve after tummy tuck surgery and it may take up to one year  (or greater)  for complete skin redraping  to occur.
 Again, let your plastic surgeon know about the changes you have experienced. Best wishes.

Swelling at 12 weeks following abdominoplasty is somewhat common.  This is particularly true if liposuction is performed concurrently with abdominoplasty. Patients generally recover well from abdominoplasty.  Most of my patients return to work in a 10-15 days.  However, residual swelling may last for up to several months.  This swelling should generally decline with time.However, I typically perform liposuction along with abdominoplasty.  This leads to improved contours, but is more invasive;  Consequently, edema persists for a longer period.  Rarely, a fullness around the navel or elsewhere on the abdomen indicates a recurrence of a hernia or rectus diastasis.  It would be prudent to follow up with your physician to determine whether the rectus plication is intact.  Good Luck!!