I had just about the same thing years ago & it was fibroids so I would take the advice and go see your doctor.
There are many reasons for this and there are many solutions.
You really should get to the docs soon tho as that type of bleeding made me very anemic.
Have you been checked for fibroids?
Hard to say. Usual reasons are Age- hormones or it could be a miscarriage...it's probably hormone imbalance of some kind but if its unusual for you, your menses are usually normal, then it's best to get a doctor's opinion. Bleeding that long can also cause anemia, so best to get it checked out.