What would cause this pocket of fat in lower back? (Photo)

It is quite possible that this is simply a result of your toro being swollen post surgery. Give your body a couple more months to heal and then reassess with your surgeon. Best, Dr. Nazarian

If this is fat, it was there before your recent surgery. Honest. Your awareness may have just started after having had your surgery, but it did not "grow there" in 2 weeks.  However, if this is truly a new bulge that definitely was not there before, it is simply fluid that has collected within these tissues via gravity. If this is the cause of the bulge, you need do nothing--it will reabsorb on its own over the next few weeks,If, on the other hand, this indeed was actually there long-term and was just now discovered by you, it will require liposuction to remove.I'd wait for a few weeks and take another photo, then compare. Best wishes!