My Last Period Was Sept 20th, My Had Unprotected Sex

my last period was sept 20th, my had unprotected sex on oct 3rd, took plan b pill on October 4th, then had unprotected sex on oct 5th. my period was suppose to start oct 18 or 19th. I took a pregnancy test and it is positive. could I have gotten pregnant from oct 5th intercourse since I took the plan b pill after the 1st time?? what are the chances that the pill did not work??