early period

I took plan b last monday and yesterday i started spotting but today ive woke up with what seems to be my normal period (same as always in flow, colour ect). Though my period isnt due for another 9 days? Can i assume this is my period so i can start my birth control pills or should i wait and see if i bleed in about a weeks time when my period would be due? I know plan b messes with your cycle but is it possible for it to be over a week early?