Please get in to see a Gastroenterologist....this is not a bug. There can be many causes for your symptoms that don't include cancer, obviously more is wrong than your GP can diagnose and shame on him for not referring you to a doctor who is specialized in this field who can properly diagnose the problem!
Without further investigation it's impossible to tell if you have cancer. I know my brother had developed ulceritive colitis but he had diahrrea for a couple of weeks and lost a ton of weight. It was diagnosed via a flexible sigmoidoscopy.
Colorectal cancer seems to hit people around 50, so it's unlikely you have it - but you may want to follow up with a visit to your GP if this isn't getting better or get a second opinion from another doc.