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.

Swelling during this time is normal. You should have your
surgeon check the site to make sure that you are not developing a seroma (fluid
collection) that may need to be drained. Also, I tell my patients that any
jogging should be avoided until we are sure that there are no issues. You had 2
procedures done, and recovery will certainly take longer than if only one were
performed. Patience is key here. Good luck in your recovery.

This is pretty common and is usually due to swelling. It may take several months before the contour of your procedure is truly noticed. You have water weight and swelling to deal with at this point. Give it time.