What Causes Heavy And Prolonged Menstrual Bleeding?

My period usually last 7 days and its pretty heavy. But by the 6 and 7th day its kinda late....i got my pd. on Sunday 15. and it was pretty heavy throught the days i was throwing alot of blot clots...the thing is we r now the 27 and im still bleeding not that much. what do u think could b the cause...should i b worried