12 weeks pregnant without knowing?

I'm 17 years old and lost my virginity in the beginning of august; it is now late October. I am on birth control, although I sometimes missed a day. I always took it the next day, but I know that that is sometimes ineffective. We also used condoms every single time, but I know they can break. Anyway, my first period was a week late, but I was in another country at the time and super stressed about being pregnant. My period has never been regular. It came and lasted 4 days. It was lighter then normal, but not just spotting, and with it came cramps, sore legs, the usual things that happen when I get my period. Last month, September, I got my period again after 29 days. It lasted 5 days. A little light, but nothing like spotting. Again, cramps and the usual. This month, I got my period again after 26 days, a little early, but with it came cramps and the usual. My point is, I hear all the time of bleeding during pregnancy and with the show I didn't know i was pregnant, i am worried maybe i am and i just don't know. what are the chances with birth control and condoms I am pregnant. were my periods really periods? i just want to be better safe then sorry. i would be about 12 weeks pregnant... i have no other symptoms. thanks