I Have Been On Marvelon For A Year And Usually

i have been on marvelon for a year and usually have spotting during the last week of the pack. I am very good at taking the pill, and haven't missed any this month and have taken them all within a 2 hour window at most. i took my last pill yesterday (sat) and usually start my period on wed, however i have started bleeding fresh blood today...i am very concerned...what could this be?