What Causes Delayed Periods?

Hi, my last period was on 19th Dec. I have a regular 28-cycle period so I was expecting it on the 15th Jan. On the 10th day since I started my period, I engaged in an unprotected sex with my fiance' but I took an EC. Since then we've engaged in sexual intercourse but he always withdraws before he cums. We always do that as am not on any contraceptive & its worked out well in the past. Today is 17th Jan and still no sign of a period. Does this mean I could have conceived?