I bloat out and gain a few pounds either in the days leading up to AF or on the day or the day after AF arrives. The muscle cells are busy in there as your hormone balance has changed to stimulate AF, so your body is working harder to regulate your hormone and electrolyte balance and retains water during this process. It's normal, I don't count it as weight gain as it's just a natural and temporary auto-regulation thing the body does. Have a google, lots of info on it.