Somebody please tell me the science of Plan B

Took plan b a week before ov and bled a lot just after ovulation. Period level of bleeding so I figured this is the uterine lining shedding early. But it's my period week and Im also bleeding just as much. What's going on here? Not the vaugue answer on the box. Are our bodies continously building uterine linings? Do I have an overactive production of hormones? I would really like to learn the nuances of whats going on but cant find definitive answers on google.