Period question

Angel

So I’m on birth control for my period and i told my doctor I did not want to have a period and she told me to take my birth control continuously and that if I did that I would not have a period and I did that and it worked for two months and then the third month I ran out of refills and I missed one day and the Monday after when I got it refilled I took two pills and then that Wednesday can any buddy tell me why I had a period ?