What Causes Blood In Stool With History Of Colon Cancer?

Hi, my Dad was diagnosed with colon cancer 3 years ago. They found a large polyp to be cancerous. They took out the polyps and did not show signs of cancer 3 mos afterwards. He was supposed to be treated for it, but he had a stroke. He has not had any issues since then, but his hemaglobin dropped 2 points over the last 2 days and they found blood in his stool. He seemed normal tonight. The nurses said they might have to give him a blood transfusion if it drops below 7. How concerned should I be?