Your body can't mobilise 28 pounds of fat loss in 3 weeks. Assuming after the operation you are not exercising intensely, your calorie burn is probably around 2000 cals per day. Typical for a non exercising male. Let's say your diet is now 1500 cals per day, giving you a 500 cal deficit so your body may be able to mobilise and use 500 cals of fat a day....
In three weeks I have lost 28 pounds. There is a real chance that I will lose close to forty pounds in the first thirty days. I'm forcing myself not to get over-optimistic about what I will lose per month in the next two to three months.
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That works out to 56 grams of fat a day or just under a pound of fat a week. The calculations are pretty simple. 9 calories is 1 gram of fat.
Of the 28 pounds loss much of that is unhealthy weight loss from stress, trauma and upsetting your body's digestive system. Much will consist of fluid drop and loss of muscle mass. There is probably quite a few pounds of contents of your digestive system included also in that loss, (which is not as bad as the fluid and muscle mass loss).
I'm glad the operation went well and that you are losing weight, but please be careful and watch your nutritional intake. I would be counting and logging calories consumed, and really making an effort with vitamin supplementation too. It places huge shock on the body to drop weight that fast.