First, it sounds like you already thought of the most obvious answer: pregnancy. While pregnancy tests are very accurate, it is possible that these can be mistaken, and so it may be worth being tested with the more accurate diagnostic tests available through your doctor
. In any rate, you should speak with your doctor because it is not normal to have amenorrhea (or cessation of menstruation).
Another common cause in young women is poor nutritional status. Menstruation is a sign that your body is able to become pregnant, and when the nutritional status is poor the body will protect itself by stopping menses so that it can focus all of its abilities and energy on just keeping you healthy. This can be common among high performing female athletes.
If you have had a cessation of your menses for a long period of time, it is important that you speak to your doctor to determine why that may be and what can or should be done. With proper discussion and a physical exam, the answer is usually straight forward and can lead to improved health on your part going forward. Please speak to your doctor in the very near future.