This happens a lotttttttttttt! Its called plateauing (sp?)
Basically, your body has gotten used to whatever it is you’re doing and isnt working hard enough to burn calories. Its either that or you’re consuming the same amount of calories you’re burning.
Have you ever tried the Body for Life program? Thats really nice for people who come across a plateau. Also, circuit training or some sort of Boot Camp program helps.
In your case, aerobics is probably a better bet. Not pilates.