I have heard it said that Atkins (or maybe low-carb in general) is a diet with a diuretic effect, so maybe the hormonal water retention associated with the menstrual period stands out more in contrast.

I've certainly noticed it more over the past few months -- my fingers sort of swell up, and the scale either stays the same or edges up a pound or two for a few days. I can hardly wait to see what my ankles do in the dog days of August BUT maybe it will be better than usual for 3 out of 4 weeks per month!