Yup, OD is correct.
When you make a rapid reduction in your caloric intake, your body goes into a starvation mode. Instead of burning calories at it's normal rate, it slows down your metabolism and you burn fewer calories. That's why it's always good to reduce your caloric intake slowly if you must make a major reduction.
Also, if you are exercising and you were not previously doing so, remember that you are also building muscle, which is heavier than fat. So, you can burn a pound of fat and gain two pounds of muscle. The bright side is that muscle has a lot less mass (takes up less space) than fat.
Remember that bottom line, your body must burn 3500 more calories than you eat in order to drop one pound of fat. That's 500 calories a day to lose one pound a week.