What Causes Vaginal Bleeding Along With Abdominal Cramps Post Intake Of Plan B?

Hi I took the generic version of plan b on the 6th of December as well as the actual plan b on the 7th. On the 16th of December I got what u would either call a period or withdrawel bleeding due to taking these pills. The blood was a pretty normal amount like a normal period slightly heavy. Since then everything has been fine however on dec 4th I began what seemed like a spotting just one spot of blood then it went away two days later the pink discharge came and has been here ever since my Breast do not hurt and I feel no stomach pains or cramps. I have taken a pregnancy test which has also come back negative. I was wondering if this could be implantation bleeding or is something wrong. Is is this the side effect of plan b. The spotting has been going on for 5 days now only when I wipe. What could be wrong?