Is this swelling or fat? (Tummy tuck)

It's difficult to say for sure, without knowing what you looked like before the surgery as well as immediately after the surgery.  It is very common to have swelling after a Tummy Tuck.  That is typically more prominent in the first few weeks, tapering off over 2-3 months.  You are still very early on in the whole healing process, and should expect changes, including improvements in swelling over the next few months.  There are also other possible causes, though, which can include fluid collections, residual fatty tissue, the structure of the underlying muscles, constipation, etc.  Keep close follow up with your surgeon to guide you through the healing and recovery process.  Take care.

“Swelling” 3 weeks after tummy tuck surgery may or may not be normal; it is best to be seen in person ( by your plastic surgeon) for precise diagnosis and treatment.   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 questions/concerns you have. Best wishes.