Hormones can make your body gain weight at any point during the month theres some people see it more the week before/after. It also depends on if you see it as an excuse to maybe be a bit more lenient with chocolate or wine (which sometimes just HAS to be done). Personally I have lost 4lb, 41/2lb, 3lb on star week before. I have also had small gains too though.
I would suggest that put a little star next to your star week weigh ins and you will be able to track your own weight fluctuations in regards to mother nature. (hope that makes sense) .x.x.