What Could Be Causing Me To Bleed Extreamly Heavy For

what could be causing me to bleed extreamly heavy for the 1st time in my life. my periods are lasting over a week and i am bleeding through tampons and pads at least 15 times a day for up to 5 days and then still bleed just not nearly as heavy for another 5 days. i dont have pain other than cramps, i have been nauseous but i am not
pregnant.