Abnormally long period?

i'm only 16, and my period has been going on for a bit longer than i had expected. usually mine are heavy for the first 2 days then it just fades away, but this time it keeps going, not heavily though. i know that it may mean pre-menopause or whatever, but i am way to young for that. i am ually active, but am not on any birth control. so i'm just wondering what are the possibilities if it does keep going on?