why am I bleeding?

I have been bleeding today and yesterday. it's less than my normal menstrual flow and I'm not supposed to get my period for 2 weeks. I did have sex a few days ago (4 or 5 days maybe) and took an econtra ez (plan b) pill that day. is my bleeding implantation bleeding, an extremely early period, side effects from plan b, or something completely different? help! 

Vote below to see results!