Open Question: what could be a reason my period would be late, besides pregnancy?

my boy and i have ALWAYS used protection, nothing has ever ripped, and even though we do use condoms, he has never come inside me. i was supposed to start around the ninth or so, but still haven't. i've asked around, and it sounds like periods just sometimes get thrown off their regular schedule? is this true? what could be a reason for this? also, i've been VERY stressed out lately, could this have anything to do with it? i'm trying to not worry about it, seeing as we've always used protection, but it is hard. answers would really be appreciated, PLEASE!