I'm copying this from a post of mine a couple of weeks ago:
I've posted before about a 10 day plateau. They might have given you some paperwork about that at you first session. Basically, your first 2 weeks are MOSTLY water weight loss because of the low-carb menu. Around that 7-10th day, your body starts holding onto water like crazy so your cells won't collapse. After a short plateau, you'll start losing pounds again. You ARE losing fat, it's just that those cells are filling with water. Hang in there and don't give up! So many people have posted the exact same question. You're not alone! It happened to me, too!
Debi in FoCo, Co