i think its really hard, and not really worth the effort, to try and decide what has caused this gain. I know for myself, I can have a terrible eating day one day, have a decent loss the next day, and only see the effects of the sodium etc the day after that! I guess it takes a while to consume enough water to retain xD So I wouldnt stress about it enourmously. If you see a steady ongoing increase in weight over a week or 2, then maybe you want to be concerned
Its really hard not knowing the cause, I know. But it'll drive you even crazier if you get too hung up on each fluctuation- which it might simply be