Hi.. Birth control pills can cause alteration in the menstrual bleeding. There is a high possibility that this would not be related to pregnancy. How ever the chances of pregnancy cannot be completely be neglected if you have been sexually active and have been practising unprotected sex.. You can take a pregnancy test but there are chances of reports turning to be false negative is also, since you are birth control pills.. The best way is to get yourself checked by a doctor if the bleeding persists...