Could Sitting for a Long Time Affect Liposuction Result?

It is common to have pain come and go after an operation of any kind. Sitting for extended periods might be a contributing factor but it is not unusual in any situation. You will be healing for many months after your procedure. The pain does not mean anything is wrong, just a by product of the procedure.
Sitting will not affect the results unless you gain a lot of wait because of lack of activity.

At 9 weeks post-op and in the absence of any post-op complications, such as seroma, excessive bruising, dehiscence (spreading) of the cannula sites, if I was your surgeon, I would counsel you to resume your exercise schedule. I would recommend starting at 50% of your normal regimen, possibly starting first with your arms and chest. You could advance your activity, as tolerated by pain, so that you should be back to full activity in 2-3 weeks.By increasing blood flow to the muscles, underlying the treated areas, the firmness which may you be experiencing will subside.
I would assume that you have already put your compression garments away. At two months, the compression garments really don't assist with more skin retraction. I would encourage you to massage the treated areas, twice daily, while bathing and before retiring, so that you can assess how the expected post-op firmness is resolving. A general rule is that once sensation returns to the areas treated, the firmness should resolve.
Reassess your caloric intake so that as you become more active, supplement your diet appropriately. Finally, involve your surgeon in your post-operative regimen. Good luck.