Generally girls are much more free about changing sexuality. Even straight girls are willing to experiment and bit and there is a much higher percentage of bisexual women than bisexual men. I read a theory that this is because when women are breast fed, they are having intimate contact with a person within their own gender and are therefore more comfortable with intimate contact within their own gender later in life. I doubt it has anything to do with her birth control pills, and if it was obviously she started liking men before than because you don't need birth control pills if you are a lesbian. Your girlfriend is fine, likely she either is bisexual or simply experimented at one time in her life.