What Chances To Pregnant While Using Withdrawal Method?

Hello, I have been on birth control pills for years and just recently have experienced bleeding prior to my period. I am pretty regular in taking it on time and hardly miss a pill except maybe one a month but never more than by several hours. I am dating someone who we were intimate on the Sept 14 and we did not use protection but did use the withdrawal method. I have been on time taking the pill before and after, the only time I missed a pill was on Sept 25 but we have not been intimate since the 14th. My question is how likely am I to be pregnant? My period is scheduled for between today and Thursday.