Usually when I have a big loss one week, I'll STS the next. i think it's my body's way to readjusting to the loss or something..
Try not to be disheartened though. Do your food diary each week for your consultant, and yourself, so that you can see exactly what you're eating. That way, you can show your C and she'll be able to point ot any errors you're making, such as not enough super free foods, or too many syns, or whatever. Often, when we don't eat enough, the weight stays on - odd to say it, I know. When you follow the plan 100% it works - BUT - don't forget you body is not a machine, and so sometimes, no matter what we do, some weeks we don't lose.
My advice would be to do a food diary this week, be 100% honest about what you put in your mouth, drinks as well, and take it to class for your next weigh in. If you haven't lost then ask your consultant to take a look and help you, that's what she's there for. If you can't wait that long, post it here and we'll take a look.
Good luck hon, you can do this!!