week 1 weigh in

Ok...one week on weight watchers. I stuck to the plan very well. I ate fruits and veggies. Drank all my water etc. The only thing I did not get in was my exercise. I guess that makes a huge difference because after the first week is over I stepped on the scale and gained 1 pound. How is that possible??? grr. I will keep at it. I am frustrated and usually that would be enough to make me give up but nope, I am going to stick with it..