What Causes Delayed Periods While On Microgynon?

i finished the packet of microgynon last tuesday as always, and my period always arrives on the Friday night/ saturday morning. This is now Wednesday and I have had no period and due to start my next packet of pills. I have been on this pill for 5 yrs and am always regular as clockwork. After 3 pregnancy tests they are all negative. I am now worried! what else could it be?