If you could spare some time, It would be greatly appreciated...

I am currently on the birth control pill Mercilon (have been for 5 months). I had Doxycycline prescribed to me at the clinic (one week long), but the nurse practitioner did not let me know that antibiotics "affect birth control". I've read about many cases in which antibiotics "cancel out" the effectiveness of birth control pills, and have also read some (one from Planned Parenthood, and a few other websites and articles) that state that only one kind, rifampin, really effects birth control.

I took Doxycycline in the middle of week 2 to the middle of week 3. I took my last active pill on September 12 (the end of week 3), which was the day that I had unprotected sex. My partner pulled out (which i know doesn't really mean anything). We also had it again the next day (the 13th) (the first day of my sugar pills), but he did not ejaculate (though I know that precum sometimes does contain sperm). I also was not aware that I had to skip my placebo pills and continue on the the next pack. I had my "period" (withdrawal bleed) on the 16th and 17th. It's usually 3 days long but this time it was only 2 days, but it was a regular flow. I had used the pill that month at the exact time every day with the exception of one time, when i took it 15 minutes late. With the exception of antibiotics supposedly affecting birth control, I should have been fine, but just combined use of antibiotics and all these scary things i'm reading online are making me nervous. What might the chances be, do you think, that I could be pregnant? I took an HPT on the 24th with my first morning urine sample, which came out as a negative, but I figured that it was too early to test anyway.

Any and all help would be great... I'm currently away from home with no family or reliable source to talk to, and am very scared...