There are a number of other reasons you could not be getting your period even though you are not pregnant. Our menstrual cycles depend on the hormones in our body (estrogen and progesterone, mainly) to work well. Sometimes, an imbalance of these hormones or the hormones that stimulate these hormones (follicle stimulating hormone, FSH, or luteinizing hormone, LH) can delay your period or make your period irregular. Often times, when teens first get their period, it can be irregular as the hormone system that regulates our periods take some time to fully mature. One example of a condition with hormone imbalance is polycystic ovarian syndrome. Other possibilities could be obstructions in your uterus or vaginal canal that are preventing the blood from getting out, but these are more rare.
If you haven't gotten your period in a while, you should schedule an appointment with your doctor to talk about the potential reasons. They can help evaluate and run tests to confirm the reason why you may not be getting your period.