Remember = muscle holds water.
The more muscle (lean body mass) you have, the more you can fluctuate in weight, depending on how depleted you are.
As an example,
After a very challenging leg workout, I can see an INCREASE on the scale of 4-6lbs.
While after a 30-45 minute HIIT cardio session, I can go DOWN 2-3lbs, depending on how much WATER i've lost during my w/o.
Women especially, hold more water, and are naturally prone to it, which is why the scale can go up so drastically, and back down within a day or two.
When you step on the scale, your weight reflects the combined total of both your lean body weight (muscle, bone, organs, fluids) and body fat weight. Two people with identical body weights do not have the same body composition; they could, indeed, have entirely different body types.
For example a 170-pound female might have 60 pounds of body fat and 110 pounds of lean body mass. A healthier, more muscular female might only have 25 pounds of body fat and 145 pounds of lean body mass.
Even though these two individuals weigh the same, one is in much better shape than the other, and by looking at the two, you'd never know they weigh the same amount.
Using the scale to measure your progress gives you no information about the body composition (fat vs. muscle) changes that are actually occurring. The scale may show that you've lost seven pounds, but it can't tell you that half of the weight was muscle and water, not fat. Similarly, people become discouraged when they haven't lost any weight, even though they have actually lost pounds of fat and replaced them with pounds of firm, fat-burning muscle.
Developing healthier eating and physical activity habits will most likely result in a loss of body fat even though the scale may indicate that you weigh the same. Learn to use other methods of determining body composition and pay more attention to improvements in how you feel, in your self-esteem, and in your physical appearance.