7 weeks post op a Brazilian Butt Lift, why do I still have big bruises all over my butt cheeks?

Too early to judge on the final result. You are still at the healing stage. The area in your lower back will be still bruised and swollen and this will take 8-12 weeks to settle. Wear your garment and keep your follow up with your surgeon.You have to give your body at least 12 weeks to start seeing results and final results can take 3-6 months depending the way your body heals

We could give much better advice if you posted a picture. You likely do not have any bruising left 7 weeks after surgery. The darker skin may be traumatic hyperpigmentation. If the areas is also hard to touch, then yes, this could be fat necrosis. You should see you PS asap as there are things that can be done to treat both.