What Causes Bleeding After Menstruation?

I was just sitting in my room watching t.v. when all the sudden my underwear felt wet. I checked in the bathroom, and my underwear is all bloody and there is a dark-ish non smelly glob in my underwear. My period ended 4-5 days ago and I did masterbate yesterday. What is the glob, is it normal and when will the bleeding stop? It's been about an hour now, but the bleeding is not really heavy.