Period? Implantation? What's wrong?

Taya
So I started my period on my regular cycle date and it lasted 5 days which is my normal, but then after 1 day off it started again and lasted another 4 days! This has never happened to me before. I do have a boyfriend and we only use the withdrawal method for birth control, we are not actively trying to get pregnant but are also not opposed to the idea. I was just wondering if anyone knows why this would happen or if it has happened to them before. Thanks. 😊