What Causes Heavy Vaginal Bleeding Post Intake Of Plan B?

I missed 3 of my birth control pills and I had unprotected sex. I bought the plan B pill and I took it about 20 hours after I had sex. I started to bleed less than 30 minutes after I took it. The bleeding stopped for the rest of the night and then in the afternoon of the next day I noticed that it had started again. The blood is pretty heavy almost as if I am on my period. I took the plan B pill on Monday afternoon and it is now Thursday evening and I am still bleeding. I am just very confused by this. When I bought the pill, the pharmacist told me to continue taking my birth control a week from when I take it. I was about halfway through my pack before this all happened. Is the heavy bleeding I am experiencing now my period? I shouldn't get my period until I finish that pack of birth control pills but I am just not sure.