What Causes Vaginal Bleeding With History Of Total Hysterectomy?

I had total hysterectomy with bilateral oopherectomy on 11/9/14. Yesterday I noticed single episode of bleeding p/ v It was less than 10 ml bright red and with a 0.5 cm clot. What could be the reason. I am not on any medication. Last contact with my partner was 4 days ago?....