Why have I gained 3 inches to my waist after having a full tummy-tuck two weeks ago?

It is likely due to swelling which is very common especially at 2 weeks out and with multiple procedures. However, you should bring this to your plastic surgeon's attention, so that he/she can make sure that it's not a seroma (fluid collection). You will see the swelling resolve over the next 3-6 months, however, for up to year you can still get swelling over the incision after an extremely active day.  The reason is that your abdominal skin needs to make new connections to drain the fluid that accumulates in tissues and that takes time (up to a full year) to occur.   Hope this helps. 

Significant swelling can actually increase the size of the abdomen immediately after in abdominoplasty particularly in a small patient.

In the first few weeks after your mommy makeover it is common to have swelling that makes you feel or look bigger than you did prior to surgery.  Compression garments wil help to get rid of this.  You should start seeing improvements shortly.  Good luck.