Fast weight loss can pull from muscle, not just fat.
Losing weight quickly can feel like progress, but the body does not only burn fat when calories drop sharply. When intake falls faster than your body can adjust, it can break down lean muscle for energy too - especially without enough protein and movement to signal that the muscle is still needed.
The issue is not that you are losing weight. It is what the weight is made of. Losing fat while keeping muscle is the goal. Losing both at once can leave you lighter but weaker, with a slower metabolism to show for it.
Do not just chase the scale. Protect lean muscle so the weight you lose is the weight you actually want gone.