Hun if you are sticking to the diet, then it can only be water retention. Many of us have had weeks where we have stayed the same or even had a small gain but it will soon sort itself out.
Stay strong as I know that this is when your chatterbox will start telling you that its not worth all this hard work to gain weight. Dont give in whatever you do, just keep taking the packs and glugging the water and you'll soon see it come off again.