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