I am 6 weeks post op tummy tuck. I wanted to know if I can have anal sex or if I would be doing harm?

Congratulations on having undergone the tummy tuck procedure. Your plastic surgeon will always be your best resource when it comes to specific instructions in regards to returning to activities. Generally speaking, best  to limit strenuous activity for at least 4 weeks after tummy surgery, assuming you are doing well and there have been no complications.  
Concerns would include an increase in heart rate and the potential for bleeding. Muscle spasm may be painful. Obviously, significant pressure on the surgical site is not a good idea either. It is not uncommon for patients to report increased swelling that occurs after vigorous exercise (including sexual activity), even several months after tummy tuck surgery. 
Generally, care should be taken during the first two weeks after surgery not to allow for elevation of heart rate, blood pressure, and/or direct trauma/contact to the chest wall. Generally speaking, most my patients are able to return to lower body exercise at the 2 week post operative mark, wait to lift/run for about 4 to 6 weeks after surgery.   
In other words, start slow and "listen to your body" as you gradually progress in intensity/duration of all activities including sex and exercise. Best wishes.

aare allowed with my patients 6 weeks post-op assuming everything is healing well and even sooner if they can be disciplined and not stress the technical portions of the procedure.  But its always best to check with YOUR surgeon since your surgeon will have to manage anything that may result from such activities.