abnormally long period/ dark blood?

Elle
Sorry if this is tmi or anything but I'm just trying to go into detail so I can get some help with this. 
So first of all, my periods are usually always on time, and usually no more than 5 or 6 days long. Also the blood is always the normal dark red color that blood is supposed to be. 
Now all of a sudden, my period comes a week early, the blood is black/ brown, and I've been bleeding for 10 days, going on 11 days. 
Anyone know what could be going on? I'm clean, no std's, no yeast infection, no uti, nothing is out of the norm. Help??!?
Update: The bleeding has been going on for 2 weeks now.