ANDY'S ANSWER: There are a number of girls that experience extreme bleeding during their mentrual cycle, especially before their periods start to regulate. Your periods might even out a bit as you get older, and there is not necessarily anything wrong, but it certainly wouldn't hurt to get checked by a doctor if this happens every period and just make sure there is no other concerns.
In the meantime, I would suggest wearing a panty liner along with your tampon so you will immediately see when it need changing without risking your underwear. You probably need to change your tampons more frequently when your period is heaviest, maybe even every 2 hours or more often if necessary.