Are you exercising too? I've taken up running over the last couple of weeks and my boyfriend who is a bit of a nerd with fitness said it can cause the scales to go up.
Muscle is being constantly destroyed and repaired then destroyed again so this will cause scale flactuation. The muscle will also cling into water to help it repair and rebuild, but after about a month to six weeks this calms down after the muscles have got used to the exercise.
I posted this quote on another thread and it seems to help. It is from Paul McKenna's weight loss book and it goes something like: "When a plane is flying from one destination to another it can be off course for up to 90% of the time, but it always knows where it's going and gets there in the end." So take heart that your body knows where it is going, it can just be off track some of the time