I just went thru this myself - only it was SIX WEEKS after my tummy tuck.
My PS gave very explicit instructions: "No running, jogging, weight training, or horseback riding for six weeks after surgery" and "Do not lift over 10 pounds for four weeks".
Like Jack said...for the first 3-4 weeks after the surgery, I really didn't feel like eating at all, or much, in the first place...on 1 January, I started walking on the treadmill (for like 10 minutes at a time) and the elliptical...just gradually getting back into things.
I'd say LISTEN to your doctor. If you have a question about WHY you can't exercise, or want to ask if you can do specific things like walking on the treadmill - ASK. Your body needs to concentrate on healing after surgery...give yourself a chance to do so!