What Causes Delayed Periods While On Althea Pills?

Morning! I am first time user of Althea. My first intake was Feb5 2015, my 2nd day of menstruation. I take 1 tablet before I go to bed for the next 21day, I took my last tablet on Feb26. My husband and I had unprotected sex, on Feb25. Im going to start the first tablet on my 2nd pack on today, Mar5 but still I dont have my period :( Is there a possibility that I am pregnant? I dont missed any tablet. When will be my period? By the way, my OB advised me take Althea to treat my PCOS. thanks in advance ;)