missed pill for the first time now cramping?

im very good at taking my birth control everyday at exactly the same time but yesterday i ran some errands and forgot my purse at home. when i got home i completely forgot about it and i felt as if it was too late. i take my birth control at 3 and by the time i remembered it was 9 o'clock. the next day i took yesterday's and today's pill and now im cramping reeeeally bad. why is this?