I am 9 weeks post tummytuck, since week 4, I have felt nausous, not vomitting just sick feeling in stomach. Could it be the swelling?

Yes. It's hard to tell what's causing the nausea. With no pain and no vomiting, it's a little far out to be having anesthesia nausea. Watch for fever and pain and see what happens before you take it up with your surgeon. You are the best judge of when it's gone too far.
Medications. Medications and narcotic analgesics are the most common causeif nausea go, lowing a tummy tuck. If your are off all of these, it could be associated with other causes. Occasionally antibiotics used for the procedure may affect your natural bacteria and cause bloating and foods tolerance or even c. Difficile colitis. I do not believe the swelling is responsible but may be a side effect of the problm.
Medicines. Can also cause nausea or odd feelings after surgery. Are you still taking pain killers or any other medications? Many of my patients tell me they feel full earlier because they have less room in their abdomen after the tummy tuck. You should discuss this with your surgeon as soon as possible.