"If you're on the birth control pill, you're most likely going through withdrawal bleeding once a month. But there's no medical reason for having a simulated period every three weeks."
https://broadly.vice.com/en_us/article/yw3p4g/why-you-have-your-period-on-birth-control-withdrawal-bleeding
https://broadly.vice.com/en_us/article/yw3p4g/why-you-have-your-period-on-birth-control-withdrawal-bleeding