A recent, persistent change in bowel habit or blood in the stool 

Bowel Care Nurses Helpline Bowel Cancer Australia 
Bowel cancer, also known as colorectal cancer or colon cancer, is cancer in any part of the colon or rectum.
Common symptoms of bowel cancer can include:
  • A recent, persistent change in bowel habit1
  • A change in appearance of bowel movements2
  • Blood in the stool or rectal bleeding3
  • Frequent gas pain, cramps4
  • A feeling that the bowel has not emptied completely5
  • Unexplained anaemia6
  • Rectal/anal pain or a lump in the rectum/anus
  • Abdominal pain or swelling7
Not everyone experiences symptoms, particularly in the early stages of bowel cancer.  The above symptoms may be suggestive of bowel cancer, but they can also be due to other medical conditions, some foods or medicines.
Don't delay in talking to your GP is you are experiencing any of the described symptoms for two weeks or more, because when diagnosed early 90 percent of cases can be successfully treated.
Blood in the stool or rectal bleeding should never be ignored. 
However old you are, you should never be told by your GP that you are too young to have bowel cancer.
While bowel cancer is more common in people aged 55 and over, it increasingly affects people of all ages.
Ask your GP to be referred to further investigations.
1. Such as looser, more diarrhoea-like bowel movements, constipation, or smaller more frequent bowel movements 
    (i.e. going to the toilet more often, or trying to go - irregularity in someone whose bowel movements have previously been regular).
2. For example, narrower stools than usual or mucus in stools.
3. Bright red or very dark blood should never be ignored.
4. A feeling of fullness or bloating in the bowel or rectum.
5. The bowel has not emptied completely after a bowel movement.
6. A low red blood count causing tiredness, weakness or weight loss.
7. A lump or mass in your tummy.
Support For You Bowel Cancer Australia