Why am I losing weight but not fat?

15 pounds is WAY too fast in 3 weeks. You should not be losing more than 1-2 pounds per week.

The MORE you lose, the more your body shuts down its metabolism and burns your muscle instead of your fat.

This is an evolutionary life-saving response .. . not enough food, their is a famine going on the body needs to slow down the rate at which it burns food. Not enough food, and the body still needs energy, so it burns muscle and saves the fat which is quick-energy is some tiger wants to eat you.

As for fat and muscle.

You can build muscle all you want, but if there is fat on top of the muscle, the only thing you SEE is the fat.

You cannot lose 15 pounds in 3 weeks and stay healthy. Or lose the fat.

And when you stop dieting, you will gain it ALL back and about another 20-30 pounds of fat.

Eat more.

Continue exercizing.

Eat about 600 calories less than your current maintenance calorie level ....no less that that many calories:

https://www.freedieting.com/calorie-calculator

Eat fewer calories 6 days of the week, and on the 7th day eat TO your maintenance amount. To keep your body from famine mode/muscle burning mode.

Poor diet, lack of physical activty, inadequate nutrition. Your metabolism is not "really fast" unless you're highly active, fast metabolism is a myth

You’re gettin rid of excess far