Effect of Morning After on Menstrual Cycle

Patient: Hi Doctor. I took Morning After Pill last week Monday (within 72hrs after unprotected sex). My last period started on the 24th Sept.(lasting four days). As far as my calendar is concerned i should have had my period sometime last week wednesday 20th. I have not had my period its Monday 24th and i am starting to worry if perhaps i could be pregnant??

Doctor: