If you have any of these symptoms, talk to your doctor - they may be caused by colorectal cancer or a non-cancerous condition. According to the ACS, symptoms of colorectal cancer may include:

  • A change in bowel habits; this could be diarrhea, constipation, or a narrowing of the stool that lasts for more than a few days
  • A continuing feeling that you need to have a bowel movement
  • Bleeding from the rectum, dark stools, or blood in the stool
  • Cramping or steady stomach pain
  • Weakness or tiredness

Source: http://resources.purematters.com/prevention/preventing-colon-cancer