Fat loss after bbl - Can my fat die because it’s looking flatter to me. Pls can you advise what I can do to plump it up again.

Hello dear!!
Thanks for the question and provided information as well. 
After a BBL, only about 70% of the transferred fat cells will survive, so it is normal to see a minor reduction in your buttocks. If you have any concerns, I suggest you to follow up close with your plastic surgeon.
Good luck :)

Hi and welcome to our forum!
The fat grafts placed two years ago are now part of you and will not undergo changes or damage due to body position. 30-50% of the graft may resorb after placement within the first month or so, but any later volume reduction is unlikely...UNLESS you lose weight. Postoperative weight loss will significantly alter the result. 
Your buttocks now also may be looking relatively smaller because the surrounding tissues are most likely swollen from your recent abdominoplasty and liposuction 3 weeks ago. 
Continue followup with your plastic surgeon and reassess your status at 6 months out from surgery.
Best wishes...