I thought of a few other things.
As already mentioned, it could also just be hormones working too. I had a gain of 1.6 pounds this week. It was my first weigh-in since 12/22. I was actually RELIEVED because this included Christmas at my in-laws for a week, being snowed in at home for four days due to two winter storms, AND just getting over my period. (Seriously, I let out a "woo-hoo".) It could have been SO much worse... like 8 pounds or something. I kind of had Weight Watchers Amnesia for those two weeks.
My suggestion is to keep following the plan, making sure to track everything. Bring your journal with you to your next weigh-in. If you have another gain, stay after the meeting and have your leader take a look at your journal. It could be that the points tracked incorrectly, etc.
You've been SO successful.. and you obviously know how to work the previous plan. While I think the new plan is so much healthier, there's certainly a learning curve involved.
Kimberly