Do not expect to lose 15 pounds a week. A loss of 1-2 pounds a week is reasonable. Losing too much weight too quickly not only increases the chances it will be gained later, but it is harmful for your body as well. Remember that muscle weighs more than fat. This is okay.
Muscle is more metabolically active than fat.
This means that muscle is constantly burning calories, even when you are not working out. Having more muscle will help with weight control in the future. Some people have to work harder than others to lose weight. Your body will take some time to change your metabolic rate, which is the number of calories you burn during a day. Cardiovascular risk factors, such as blood pressure and cholesterol take a long time to improve as well.