Yep, it's most likely just your body adjusting. One of the things with WW is that you end up eating like you're supposed to. And you end up eating the things you're supposed. Once your body adjusts and "knows" it will always have enough food it will stop holding onto the extra. It doesn't take long to adjust, usually about a week. A little gain once in a while could actually be a sign that you're doing it right. Don't foget to adjust your points if you're on flex at every milestone. That will make a big difference, too. And sometimes, you just gain a little. We all do from time to time. It could be due to so many of the things that Nina mentioned. The most important thing is not to beat yourself up and keep moving forward. You've only failed if you give up now.
I also gain about 1-2 lbs. of water with my period like clockwork.
You're doing good! You made it 2 weeks. Now it's time for week 3. Like Nina said, just stick to the plan and as the weeks go by you'll find it becoming easier.