Hi,
It's common for young women who are in their teens and during their first year of having periods to experience irregular menstrual cycles. The average menstrual cycle is 28 days long from the 1st day of one period, to the first day of the next period. Sometimes your period might be on time, and sometimes your period can be early, or late. There are quite a few things that can cause irregular periods, for example: stress, medications, changes in hormone levels, poor diet, an illness, etc... If you aren't sexually active and you haven't missed more than 3 months of having periods, then you shouldn't be worried right now. Try not to worry too much about it, because stress is a big factor when it comes to delayed periods. I hope this has helped :)