It could be endometriosis, thyroid related (hypothyroid causes heavy bleeding), fibroids, any number of things. Best to get it checked as soon as you can.
I have had extensive heavy bleeding and mine was due to Hypothyroidism and also a large vein which had burst, and needed an operation to seal.
I would get to the Dr. If it's been more than a month and your still bleeding. There are several things that can cause bleeding. My mother had the same thing happen and her case was different she ended up having cervical cancer... I am NOT saying that's what it is, but it's possible there is something going on the Dr's need to look at.