Is it possible for your butt to flatten 8-9 weeks post op due to prolonged sitting?

Thank you so much for share with us your concern. You need to avoid this kind of trip, with 8-9 weeks your body is still inflammation and you need to be careful with the first 4-6 months after surgery. However, your butt should not be flattened by that trip you made, in case you have any doubt, please contact your Board Certified Plastic Surgeon. 
Best of luck!
Dr. Quiroz 

Usually by about 6 weeks what has "taken" has "taken".  It is unlikely at this point you caused damage to the fat. You are also likely still swollen, and that may take several more months to settle completely.  It is a good idea to not sit on one area for any long period of time and continuously shift positions.