Breakthrough bleeding has several potential causes, but on the birth control pill, it is most likely an insufficient level of replacement hormone resulting in an unstable endometrial lining. The usual remedy is a change in pill to a different level of hormone combination. However, it is possible for infections like chlamydia or gonorrhea, abnormal cervical or uterine tissue, or pregnancy to cause bleeding between periods, so it is best to have an evaluation with your women's health care provider.