Hi there Kerry! 

The fact your blood work came back fine is reassuring, but doesn't mean you're cancer free, it also certainly doesn't mean you have it either. CT scans take roughly 2 weeks from what I know to get results from them analysed. I'm sure some cases are faster, and some are slower, however I believe if they did find something on the scan, they would tell you firstly, then ask you if you think an MRI or Colonoscopy would be better, of course the doctor would also recommend what he/she thought would be best! It would be unusual from what I know ( Which isn't much, I'm no doctor) for them to go ahead and schedule a colonoscopy only a week away without telling you what it's for, what I think is more likely is that your doctor has decided to book both at the same time, but the colonoscopy was only available a little later!