Birth Control?

Hey, everyone! I'll start this question with a bit of information about me. I'm almost eighteen years old, happily taken, and actually still a virgin! However, with all of these things I have terribly painful and obnoxiously heavy periods that are NEVER regular. I always ask my mom what to do and she always tells me that all the doctors would do is put me on birth control. Not that I need it for the prevention of babies right now, but I am considering it if it will help with the periods. My ONLY concern is that it will make me fat. I have low thyroid and so I already have a tendency to weight gain, but I am quite self conscious. Does anyone have any advice on what to do or has anyone not gained weight from birth control? Any advice is appreciated. Thank you!