I am 14-years-old and not sexually active. I have had my period for about 1 1/2 years now and to this point it has always been regular until now. I am 5 days late, and I don't know why. Should I be concerned?
Generally, women can expect the unexpected with their menstrual cycles for the first 5 years of having them. Even then, it's still common for young women to have erratic cycling into their early twenties. That means you're normal. Even if your period has been totally well-behaved before, it can turn at any time for the next few years. The simple explanation is that with all the developmental changes going on in your body right now, your body is still trying to get accustomed to and make sense of all the surges of follicle-stimulating hormone, luteinizing hormone, estrogen, progesterone and every which way gonadotrophin your body is churning out right now. Don't worry, I won't quiz you on the names.
If your period vanishes for several months, by all means mention it to your physician. But a five-day delay is nothing to worry about for the moment.
For more information, read M.I.A or, Dude, Where's My Period?