What Causes Vaginal Spotting And Stomach Cramps While On Birth Control Pills?

I am a 20 yr old female. I have been on the pill for about 5 years... here are my complications:
-last month I went to the doctor because I almost blacked out at work(7/31/10), had not been sexually active for about a year. The only thing they could diagnose me w/was a UTI, so I took my cipro antibiotics. About a week later I became sexually active again and had unprotected sex.(8/10/10) I was scared that I might get pregnant,about 17 hours after the sex I took the plan B pill(8/11/10). A few days later(8/12-8/25) I had unprotected sex again/up until about 2 weeks later when I was a few days late starting my period(8/27). I usually start on the tue. or wed. of my inactive pills and I started on thurs. It was heavy thurs. but after that it was somewhat lighter than normal until sunday. I started my new pack of BC pills(8/29) and around last weekend(9/2-9/5??) I had to double up my BC pills within a day of each other. I was still having unprotected sex(8/29-9/10). Around 5 days ago I began spotting and light cramping here and there for about 3 days. The spotting was light brownish and kind-of glossy, but at times it was light pink. Its been 2 days (9/12)since I have seen any spotting. It is still about 2 weeks from my scheduled period (9/26) and I was just wondering how likely it would be for me to be pregnant and any other advice you have to offer. thanks