Took Birth Control Pill And Missed Periods

Patient: I just recently got prescribed for the pill and my doctor told me to take it for the first time on the day my period starts. My period is always regular and as it was meant to come on friday i took the pill on that morning (a white sugar pill) my period did not end up coming that day though or the next following days but i continued taking the pill. My period has still not come though and is now 3 days late. It is highly unlikely that i am pregnant as i have not had unprotected sex but i dont know why else my period hasnt come yet.