What I have noticed by weighing myself every day (a practice, by the way, I generally frown upon.)
I have noticed that at a particular time every month I gain three lbs for no reason at all, and then once that particular time is over, I loose it again (and some more to boot.)
If I eat even a smidge above 100 g of carbs in one day, I gain weight. "Maintenance" carb allowances for me are 50-100, and I have to be in ketosis to lose. Of course, once I get to a better weight, this may change to a more normal pattern, but right now this is what reality is for me. It explains SO MUCH about my body, about how it works, and about why weight-gain is so effortless when I eat so little historically.