Being irregular during the teens is very normal even if you were previously regular. Women of all ages can have longer than usual cycles or late periods. Things that do this are stress, worry, changes in routine, weight gain or loss, illness, trauma. Even going on vacation can mess up our hormones. So, I would not worry. If you don't have one for three months, that is the time to call a doctor as they like women to have a period at least every three months. good luck