The post above is correct! Your period will probably do all sorts of things like this until your body gets used to it. A late period every once in a while isn't anything to worry about unless it's ALWAYS late. My periods were always late when I was a teenager(sometimes I would go 4 to 6 months without one!) and my doctors eventually diagnosed me with a condition called PCOS. It can take a few years for things to become normal, and like the poster above said, stress can wreak havoc on your body. Unless your periods are late every time, you're perfectly normal!
There are other reasons a woman's period could be late besides pregnancy. The female body can be very sensitive to changes in our lives. It could be stress or subtle changes in your body as you grow. You are still young and your body is probably still growing and adjusting to growing up. I'm sure you are just fine and it is nothing to worry about. Just relax. You have the rest of your life with this monthly "gift" and honestly stressing about it being late could make it wait longer. It'll be just fine. If you are still worried try to talk to an adult woman in your life who you trust. Your mom, an aunt or older sister, etc. We have all been through this. Who ever you talk to will surely ease your mind better than a faceless woman behind a keyboard. But, I'm sure you're fine.