I Am Currently Concerned I May Be Pregnant, But I

I am currently concerned I may be pregnant, but I am not trying to get pregnant. My fiancé and I last had intercourse on October 17. I am on birth control and we also used the withdrawal method. I had a period on October 19-21. From November 5-10 I had light spotting, which may have been because I missed a pill on October 27, two days after I started a new pack (October 25). My period is due this week, but I am concerned because my breasts are slightly tender; however, that could be in my mind because I'm so concerned about being pregnant. What do you think the likelihood is that I am pregnant?