What Causes Delayed Menstruation When On Birth Control Pills?

I've recently changed from being on Cilest for 4 years onto Marviol. This is my second month taking Marviol and i'm on day 4 of my 7 day break and i've not gotten a period, i haven't had any intercourse in the last month so i know i'm not pregnant but is there a reason my monthlies have not arrived? This has never happened before, i always have a bleed during my pill free week