Athlete on Birth control pill???

So I am 17 and I swim competitively and will swim in college next year. I have been dating my boyfriend for almost two years and we wanted to wait a while before coming sexually active. We want to have sex for the first time next weekend. He and I think it's smart to use a condom, but we would feel better about me being on the pill too. My mom won't put me on the pill because she thinks it will impact me in a negative way and make me gain weight... She doesn't know I want to have sex. I asked her about the pill because I cramp really bad when I am on.  Can someone clarify this and let me know if a condom is safe enough and if the pill makes athletes gain weight?