By the time symptoms appear in a patient with colorectal cancer, the tumor may have already spread. An early symptom that may be associated with colorectal cancer is a change in bowel habits, such as diarrhea, constipation, or a narrowing of the stool that lasts for more than a few days. Alternatively, patients may experience a feeling of incomplete bowel emptying following defecation, rectal bleeding with bright red blood, blood in the stool, which may make the stool look dark, cramping or abdominal pain, weakness and fatigue, or unintended weight loss.