There are lots of reasons for your period to be late, especially if you're 13. You've already said that you aren't sexually active, so you can rule out pregnancy. That's good.
But it could be anything from losing a significant amount of weight to lots of stress to exercising a lot to a specific physical problem. The best thing we can tell you is to get an appointment with a gynecologist (we recommend girls start doing this once they've had their first period) and see what they think, to make sure there's nothing wrong.
~lemming, Scarleteen Advocate
this is what you get for liking it.
"Sebastian, you're in a mess, you had a dream they called you king of all the hipsters - is it true or are you still the queen?" --Belle and Sebastian, "Put The Book Back On The Shelf"