Many, many things can cause a period to be late. Stress is the number one. Your period is late, and then the stress of it being late makes it later, and it never comes. I am a virgin
but I have missed months at a time because of stress. After a close friend died, my sister did not get her period for almost a year - but obviously wasn't pregnant (though she worried about it for the first few months). Medications can also do that, as well as weight loss or fat loss (too much muscle). Quite honestly, some sexually transmitted diseases can cause it as well. There are ANY number of reasons, so don't freak out until you see a + sign on the pregnancy test. But she still MAY be pregnant - tests aren't always right, or it might not be far enough along to tell.