I've read several articles in Oxygen magazine about the fitness models who work to stay within 10 lbs of their goal and then when they are pre-show they whittle it down. Even those beautiful women who look amazing don't keep it down to that "number" on a regular basis.
It is also possible that you aren't eating enough for the level of working out that you are doing. If you track your fitness minutes on SP you'll see a total weekly estimated calorie burn. Take that number and go to the start page and in the box on the left called My Progress and Goals click on the red word change under Calories Burned. Put your number in and save. The system will recalculate for you what your daily calories should be. I tend to stay in the 1,600-1,800 range for my level of fitness minutes.
Your metabolism will slow down if you're not consuming enough and your body will hold on to what it's got even more. In the end though you need to find a way of eating and working out that you can maintain for a lifestyle permanently. Make it fun, and make it last.