Changing from mini pill to combined oral mid packet? please help!?

Hi, my doctors messed up my prescription for the pill and instead of putting me on cilest (combined oral pill), they put me on cerazette (mini pill). I called them and explained they had done so and they prescribed me my original pill (cilest). They said to start it straight away (midway through the other pill i was taking)... will this cause breakthrough bleeding? i really want to know because its my anniversary next weekend and i'd really rather not be bleeding then. any help appreciated.. thanks x