Hi,
Missing a period or having a delayed period ( 'irregular periods') can be considered normal for young women in your age group, and for much older women. An irregular period can be caused by various things in your life, such as: stress, poor diet, miscalculating your next period, illness, changes in weight, medications, etc... Since you are not sexually active, this isn't something that you should be too worried about. Keep in mind the more you worry about a late period showing up, the longer your period can be delayed. If it continues to bother you or stress you out, speak with your doctor about how you can regulate your periods during your next appointment.
Take Care