Weight gain on the pill

So I recently started birth control and my sister and a few of my friends assured me I would gain weight. Now I used to be kinda chubby but was able to loose weight from really watching my diet. I also have a huge fear of gaining weight due to being bullied over my size in the past. So is it completely certain that I will gain weight from the pill, or does it vary from woman to woman? Any first hand experiences?