What Causes A Delayed Period After Intercourse?

Hi My boyfriend and I just had sex for the first time (for both of us) and now my period is a week late. When he first went to put the condom on he realized it was going to be upside down so he flipped it over and put it on. I'm worried that precum may have gotten on the condom therefore getting me pregnant. However... When he went to 'go in' he missed 2 times so that may have possibly wiped it off, also he didnt go in far at all due to pain. My periods are late sometimes and i've been stressed the last two weeks because of this so that may be another reason for its tardiness. What do you think?