What Causes Delayed Period While On Birth Control Pill?

My boyfriend and I had unprotected sex on monday, a day after I took my last birth control pill. Since then I have not gotten my period (it's friday today). Could I be pregnant? (it should be noted however, that I have been anxious about the matter every since we had the sex, so I was wondering if missing or the lateness of my period could be due to my mentality? or stress? Thanks