While it's not impossible to get colorectal cancer at 21, it is very, very uncommon. And there are far more likely reasons for your symptoms. The thyroid level being high could explain a fair amount of it - diarrhea and more frequent bowel movements are one effect of that. Stress or anxiety could also explain most of your symptoms. As could IBS, possibly something like lactose intolerance, etc. 

Have you seen any blood in your stools or had any weight loss? Was the fecal sample a FIT test? If it was and it came back negative, then colorectal cancer is, I believe, very unlikely indeed.

Also bowel movements more than once a day are still within normal limits. Yes, a change in bowel movements can be a sign of colorectal cancer, but there are far more likely explanations.

A year ago, I had rectal bleeding, some weight loss (about a stone in a year), occasional loose stools (maybe once a week) and trapped wind. The specialist said that as I was only 40, the odds of cancer were extremely low and I would be "the unluckiest person in the world" if it turned out to be that. You are little more than half my age and haven't mentioned bleeding or weight loss, just the symptoms that are consistent with far more common conditions, so I would say the odds in your case are even lower than in mine.