What Causes Discharge Of Blood Through The Nipples?

I had a diagnostic mammogram and ultra sound on December 17th 2014. I have very fiberistic breast tissue and have multiple cysts. On Sunday I had a bloody discharge from one of my nipples. It is now Wednesday and with not too much of an effort, the blood is still there. What could this be? I am actually going for another mammogram and ultra sound tomorrow. Please help.