What Causes Heavy Bleeding With Blood Clots During Menstruation?

I'm 14 years old and my periods usually last between 4-6 days at the most. They are usually light to medium. Tomorrow I'll be going on my 8th day. This time my period is very heavy and a few blood clots. I've been having periods since I was about 12 years old. They are usually around the same time every month but this concerns me because I've never had it happen before. Is it normal??