What would make you start your period a week early if you were on birth control pills?

Usually breakthrough: The most common cause of this would be progestin breakthrough bleeding. The birth control pills can sometimes do such an effective job thinning the lining of the uterus that the lining becomes fragile and unstable and can bleed or shed unpredictably (and during the active pills, not during the placebo pills at the end of each pack.) could also be a cyst, polyp, fibroid or thyroid or other hormon.