Oh that sounds frustrating

But as weight loss goes, it's kind of hard to predict things from one week to the next. Usually an initial large loss is due to dropping excess water, along with some fat. If you haven't been exercising and start a strenuous exercise program, your muscles can retain water. So maybe you lost a lot of water with FS then regained some water with EFS. That's just a guess

You could be losing a steady amount of fat along the way.
Losing 15 pounds in 3 weeks is a pretty extreme goal and not possible for most people. Losing 1/2 pound, 1 pound, or at most 2 pounds per week is a better bet for most people.
At least the scale is going down, so you're doing something right