21yr old male, could this be colon cancer?blood on stool,rectal bleeding(bright red/darker at times,) painful bowel movement, big & hard stool, etc.

Constipation/blood: It isn't common in 21 year old males, but always a possibility. It may be a proctitis, colitis, hemorrhoid from constipation, or anal fissure from passing large, hard stools. Try miralax (polyethylene glycol) and colace over-the-counter daily until your stools are softer and less painful to pass. See your primary care physician or Gastroenterology specialist for an exam to find the cause and recommend treatment.

Perianal fissures: From your description it sounds like you have some fissures(tears) in your perianal area due to constipation. See your primary care doctor and get it checked out. You may need to use some stool softeners and topical ointment to help relieve the symptoms

Chronic constipation: Chronic constipation can lead to hemorroidal bleeding and fecal impaction Once the stool is hard and large, medications are generally not effective initially. Treatment consists of preventing constipation through diet, bowel retraining and stool softeners. To get things moving at the beginning of treatment, Fleets enema may be necessary. Without a strong family history, cancer would be uncommon.

Hemorrhoids: I think its more likely hemorrhoids. It would be great if you could get a stool softener. After the stool is softened talk to your family doctor

Unlikely, but: This has already been expertly answered by several colleagues. I would only add that if you have a strong family history of cancer (colon or otherwise) or family history of polyposis, you should talk to your doc. Some inherited genetic traits can lead to cancer even at ridiculously young age. Best to you.