Keep in mind that not all of your initial losses are fat pounds. In the realm of bariatric surgery, fat loss is what matters.
The nurses at the hospital or surgery center probably infused several bags of intravenous fluids into you before, during and after your procedure. These IV fluid bags weigh a substantial amount and make it appear as if you've gained/lost more than you truly have due to water weight.
Also, you lose weight and volume as swelling decreases.
Don't worry...this will probably be the last week you ever lose 16 pounds.