Hey
I've been on the pill for several years now and this has happened several times. Try not to worry about it too much. Like said above, it should push it back but sometimes your body gets a bit confused and makes you get your period. Also, is it your full on period or lighter like spotting? Sometimes if you miss a couple days you can spot, and skipping the week off and missing a few could make your period start. That being said, this is normal, don't worry about it.
PM me if you have any other questions!