birth control weight gain

I have been taking the pill for 4 months and I have gained about 20 pounds. But at the same time, I am a college freshman, so I thought it might be the freshman 15. I have been working out at the gym 3 times a week pretty regularly and eating in moderation, just 3 meals a day and minimum snacking. Should I change my birth control and if so, to which other birth control?