took plan b and now...

My boyfriend and I had sex on June 10th... the condom broke. I had one of my friends buy me Plan B the day after (june 11th) except we were both so busy I couldn't take the pill until June 14th! It's been about a week since I took Plan B and I'm bleeding... almost like I'm on my period. What is the cause of this? Is it my period ? Does this mean the pill had worked ? I've never took plan b before so I have no idea what to expect from it. Please let me know yalls experiences. Thank you.