Stalls happen to everyone. At three weeks it seems all of us had one, but it is usually your body adjusting to the surgery. Your healing it is trying to figure out where it's food storage container is and why it has had mostly liquids for the past few weeks. After it regulates itself out it will start to lose again, but this time in the process is frustrating and it will happen again and again thru the journey, but it is normal.
I would define a stall as not losing for weeks or months, but my major one wasn't a "stall it was a readjustment. I was still losing body fat, cloths still fit better, but the scales didn't move. Lean mass is heavier and takes up less space then fat so even though the scales weren't moving I was improving. Scales can be deceptive as most just measure pounds. A true body analysis tell the whole story.
I have stopped losing weight three times so far and for weeks at a time, but my Dr. said he is happy with my progress and it will naturally continue when ready.