What's the likelihood that I'm pregnant?

I'm currently taking birth control pills but the last time I had sex was a day before my last period, a few days after that I finished the month of pills and was supposed to take the new months pill on a wed. but my gyno told me that if I were to take it before 24hrs. of your last pill then I should be fine. I ended up not taking wed.'s pill until thursday afternoon. (This is four days after I had sex) We did not use a condom and he came inside of me. Knowing I was on birth control, I thought nothing of it at the time. Now it's two weeks before I'm supposed to have my next period but I'm spotting and cramping. I know it could be breakthrough bleeding, but I also know there is a chance it could be implantation bleeding. I'm just not sure on the likelihood of it. Any advice?