What Are The Chances Of Pregnancy After Taking Plan B?

I took plan b an hour after sex (9 days ago), and now I believe I have my period. But I'm worried it could be implantation spotting? It has lasted for two days now, its definitely blood red and it seems simply like a light period. I have my usual level of heavy cramping as well. A pregnancy test turned up negative this morning, but I'm worried it's too soon for it to tell. Could this light "period" be caused by plan b? This is the usual time in my cycle for my period to occur.