Pregnancy is the most common reason for a missed period, but it isn't the only reason. In this video, SciShow explains a few other reasons you might skip a period.
Of course, your period happens when the lining of your uterus sheds once your body realises there's no egg to fertilise. However, if your body doesn't have enough energy available, this process might get disrupted, and that can happen if you've been exercising a lot, for example. The video explains:
Which would explain why amenorrhea is most common among ballet dancers, gymnasts and long distance runners. One study at Penn State University found that about half of exercising women experience at least some kind of change in their menstrual cycle. And a calorie deficiency can stop periods, too… exercise and too few calories both affect the menstrual cycle because they have a similar effect on the body: energy deficiency.
In simple terms, when your body lacks energy, it saves it for certain functions, which doesn't include reproduction. For more detail, check out the video above or head to the link below.
Why Did You Skip a Period? [SciShow]