What Causes Vaginal Bleeding After Having Plan B?

My period started on February 5. I had sex after a night of a few drink about five days after my period ended on February 10. Just to extra preciouses I took Plan B the next day I also had a birth control consultation the next day. I was advised to take my birth control the day it started or the Sunday of it starting. However, I believe my period has started early due to the Plan B this has happened both times that I have taken Plan B. So my question is should I take my birth control, wait till next month, or write this down as something else like spotting?