Butt Gets Sore from Sitting After Brazilian Butt Lift?

I think you are progressing quite normally after this surgery. The soreness will improve and you are NOT killing your fat cells by siting at this stage. There is a normal amount if absorption that will happen regardless

Brazilian Butt Lift recovery time is dependent on how the patient’s body reacts to the procedure. Patients usually return to work after a week but you should not sit without the use of cushions for at least two weeks and you must wear a compression garment for several weeks after the procedure. You should wait until at least 2 weeks after the procedure to engage in sexual activity. Swelling and discomfort normally fades at six weeks. After about a month you will be mostly healed and you can get a nice idea of the permanent results after around six months.

It's great to hear that you have been so conscientious about sitting after your brazilian butt lift. You are still recovering from your surgery so it's normal that you feel a bit of soreness from sitting. You'll feel better as you progress in your recovery.

It is very normal to feel that soreness while sitting for the first few times especially after going so long without sitting.  The more you do it, the better it will get.  At 11 weeks, sitting in a progressive fashion should not kill your fat cells.  Keep doing what you are doing.  It will get better.