I might be wrong, but I always thought that drinking water can help you not retain water *ducks*

I did not gain any weight when I started my Pill when I was 18. I was active and outdoors, but ate like a pig also. I was just lucky, maybe. Also did not gain weight after I had dd and went back on the Pill, but I was more consious and making an effort to lose baby-fat

I don't think you need to worry about it. Just eat healthy and stay active. Like Bec said, you actually have to EAT more to gain weight