My Husband Had Colorectal Cancer Stage 3c. He Has Had

My husband had colorectal cancer stage 3c. He has had two clean CT scans since his treatment was completed, once just a week ago. However, he has been passing blood into his colostomy bag-sometimes black (liquid), sometimes maroon or red blood, and he has also passed what he explains as little black hard rocks. He has been having nausea and vomiting off and on as well. He has also had fatigue and just a general ill feeling with the other symptoms. Do you think the CT scan could have missed any metastatic cancer? Or could this be symptoms of a totally different issue?