Aw hon, please don't lose heart. My leader once said this is actually very common as the body is adapting to the change, plus sometimes a pre-joining-ww binge catches up with you at this stage, it can take a while to show.
Of course it could be hormonal, or indeed down to the excercise. When you first start to excercise, your body stores extra water and 'fuel' (protein, glycogen, etc) in the muscles as it wants to make sure its well prepared for future excercise. Once it gets used to the amount of excercise, it gets more efficeient at creating fuel for energy, so doesn't need to store as much. Hope that makes sense, and helps you realise that you need to keep up the good work - honestly, its well worth it!!! You are doing great xxx
Don't worry in the slightest - when I did SW for the first time I put on 1/2 lb in my 2nd week - I'd tried so hard I cried my eyes out in class! I went on to lose 3.5 stone in 5 months and regularly got Slimmer of Week/Month!