What Causes Abnormal Menstrual Bleeding?

Today I started bleeding but I am far away from my period. I would say it was my hymen but that would mean that this would be the third time it's broken and I don't think that can happen? Since I am sexually active I'm not sure if pregnancy could be an issue, considering I took a Plan B pill about a week ago and the latest I've done is oral, so I'm just conserned about the bleeding and what it might be?