This is one of the most obvious causes of weight gain within a day for women. During menstruation, the hormonal changes women go through (more specifically, the estrogen and progesterone hormones fluctuation) may lead to a sudden weight gain.
Besides, some women also experience water retention and bloating, which can further accentuate weight gain.
These extra pounds usually go away when the menstrual period ends, only to reappear the next month or during ovulation. It’s cyclical, so you better get used to it.