12 weeks post. Could this be fluid, swelling or fat left behind?

It may still be swelling.  This can persist for several months, and tends to fluctuate with activity level at this point in recovery.  Final results are not seen for 6-9 months from surgery.  I encourage you to discuss your concerns with your surgeon as he/she is most familiar with what was done in surgery and how you've been progressing through recovery.

Hello dear!!
Thanks for the question and provided information as well.
You are still early on the healing process, it can take up to six months to see your final results, so please be patient. At the stage you are in, inflammation s very normal.
If you have any concerns, please follow up close with your plastic surgeon.
Good luck :)

It would be difficult to give an accurate assessment based on photos. Your best course of action would be to schedule an appointment with your board certified plastic surgeon for an in-person examination and evaluation.