i had a really scary time last year, the bf and i had just become sexually active, and we were using protection but unsure of it. i was about a week late for my period, and let me tell you i'd never been so scared. luckily it was caused by recent physical trauma (sprained ankle), change in diet (couldn't eat properly because i couldn't get food because i couldn't walk) and scholastic stress.
another interesting thing: your period is late, so you worry, and sometimes that worry can make your period even later. ain't being female grand?