Ditto what Lynne said, but a couple of other thoughts:
First, a little diet math. In order to gain a pound, you need to eat 3500 calories over and above what you need to maintain your weight. For the average woman, that's about 1900 calories per day. So, unless you ate about 2900 calories per day last week, that is not a weight gain. It could be muscle, it could be water retention. As I've lost weight, I've discovered my body has gotten much more sensitve to salt. Also, certain foods also make me gain. I get a guaranteed gain of three pounds the day after eating any kind of Asian food. It does away after a couple of days. That's life. It could be your scale, it could have been the time of day you weighed, you could have had more food in your colon than the last time you weighed (to put it delicately). The scale fluctuates, that's life. Unless you've been overeating, it's meaningless. Don't freak out about it.
As for the plateau, go back to weighing and measuring your portions. My leader makes a good point that as you get comfortable with the plan, you may stop weighing and measuring because you think you know what a portion is, but over time, the portion size will gradually creep up and you won't even realize it until the needle stops moving down. So give that a try.
Also, consider if you're eating too little. If you are, your body is going to hang on to the extra weight. If you've been exercising a lot and consistently eating your minimum calorie allotment without using your activity or flex points, try eating an extra four or five points a day and see if that makes a difference.
Conversely, if you're eating all your activity and flex points, try going without for a week to see if that makes a difference.
Try the Wendie Plan. I know it's on one of the WW message boards here somewhere. Basically, you vary your points intake every day. I've never tried it but I've heard it works well and it's a good way to have some different patterns to kind of jolt your metabolism and get it going again.
Finally, I've read that sometimes your body just needs time to adjust to a new weight. I read one book once that recommended only losing 10% of your weight at a time, then taking a couple of months off to maintain, then losing the next 10%, etc. Maybe your body is just taking a break.
Basically, whatever you've been doing consistently the last couple of months (whether it's your eating or exercise plan) mix it up a little. And if nothing else, just have patience (I know, easier said than done). If you're doing what you need to do, eventually, the weight will start coming off again.
Josie