Weight gain with birth control pills?

I’ve heard it is possible. If it’s happened to you, how did you know you were gaining weight from the pill and not just gaining weight due to your own eating habits or lack of exercise. I’ve been on the pill for almost two months and this first month I was a pms nightmare, now I’ve noticed my stomach is getting bigger and I think my butt- but I’m not sure if it’s because of the pill or not. If it is from the pill, if I go to the gym and eat healthier is it easy to fix or it is it a nightmare?