I wouldn't start worrying too much about it now, since it's already happened, but try to not miss anymore. If you do, I'd suggest extra protection. Can you say "russian roulette"? I've known more than one lady who have gotten pregnant while not missing any pill. I know women off the pill completely for years (usually trying to conceive) and not get pregnant. If you're wanting to avoid pregnancy, put your pills right next to your tooth brush. If you forget at night, hopefully your remember it in the morning!
you can always get pregnant when you have unprotected sex. Although the likelyhood of getting pregnant on your period is slim, it can happen. Did you stop taking your pills two days early and that's why your period started or did you miss one day here and miss one day there? If you've been on the pill for a while (say 3+ months), you're probably safe.