Why would i need to be screened for colon cancer whenmy blood count was normal?

I have pain in my left side above my groin. I was sent for a CT and had a CBC done and everything was normal. I am only 35 and my doctor said I need to get a colonoscopy to diagnose what is wrong with me. I don't understand why? Isn't it true if I had cancer or colitis it would show up in my blood cont or CT? Please help me understand what they can find during a colonoscopy that they couldn't find in a cat scan or blood work?