Sarah,
Don't worry! It could be water weight, due to extra salt or your body just naturally holding on to some added fluid. It would also be caused by different clothing, if you wore heavier pants or long sleeves or something.
If there is really no reason why you should have gained weight, wait and see what happens next week- chances are that if you have a good week this week, you will lose it and more next time you weigh in!
I once gained 8lbs in one week- the lady weighing me almost fell over! Then, the next week I lost almost 10. The body does some weird things!!!
The important thing is for you to not panic, and keep on going!
I think it is good if you are working foods like chocolate and chips into your pts. You need to do the program in such a way that you can make it a lifetime commitment. If you cut out all the foods you love, you are just asking for trouble later on!
Good luck, and don't worry!
Tara