There could be multiple reasons. You should talk to a doctor or other appropriate medical personnel.
You didn't say what kind of birth control you were using. One reason for missing your period is that you're pregnant. Or, if you were taking hormonal birth control (pills, the ring, etc.), the changes as your hormones settle may affect your cycle. This is not the kind of thing you should try to resolve by asking unqualified people on the Web for opinions -- check with medical personnel.