I have stopped using birth control injection 3 months ago. but haven't started my period yet. what could this be?

It may take time: There are many forms of injectable birth control that last different amounts of time. Medroxyprogesterone ( known as Depo Provera) is a common injection that generally is administrated every 3 months. After discontinuing this form of birth control it can take a very long time (6-18 months) for your regular period cycle to resume, and even when they do restart, they may be at irregular intervals.