On birth control pills and no period?!?

I have been on birth control pills for over a year now. My gyno wanted me to try them because I was having problems with long, heavy and painful periods. Things have definitely been better on the pill and my period is much more regular. I take the last active pill on Saturday nights and by the first half of the day on Tuesday - I begin my period/withdrawal bleeding. It is Thursday night now and there is no sign or symptom of it coming. Should I be concerned? What would cause this to happen? I have never missed a period on the pill. Thanks!