What Causes Delayed Periods While On Birth Control?

about a week ago i went to the doctor to get on birthcontrol, this was a wed. she told me to start taking my pills that day which i found was weird because i was always told to wait until after you get your period but i started taking them on friday, missing a couple days here and there since, and changing the time. i was originally supposed to get my period yesterday w/o being on birthcontrol. should i be worried? or should i just be waiting to take the sugar pills?