First off, take a deep breath. And then another one. You are making giant leaps here. My first assumption would not be cancer, it would be hemorrhoids. And no, the ED would not schedule your for a Colonoscopy or scans. You are not presenting with an emergency. The ED gets you stable and either admits you or sends you home. Since you do not have any emergency symptoms, they would likely triage you and send you on your way.
Your doctor is the place to start. They can see you, and figure out where to go from there. A colonoscopy is a great idea but usually takes a few weeks to schedule. You have to do prep work for it and it is not done spur of the moment. See your doctor tomorrow, get some advice on what to do next. You will probably get in with a local Gastroenterologist long before you will get in with Dana Farber. And as for advanced cancer and the signs, sometimes there are some, but a lot of times there are zero. My husband had zero symptoms until about 3 days before diagnosis. No blood in his stool. Just constipation and Upper abdominal pain (from the liver mets). You are one of many that come here terrified of their symptoms and that they mean there are cancer. It rarely turns out to be cancer for them. Follow up with your doctor, listen to your body, and try not to make giant leaps. I wish you the best!
wife to DH, dx 8/15 stage IV @36, numerous unresectable liver Mets
resection and HAI placed 12/15
Liver resection 5/19/2016 15-20 mets removed (surgeon lost count, but it hopeful he got it all!)
Recurrence 7/2017 4 Liver mets and one possible lung met
Radiation on liver Mets 12/17
Lung met 10/18
VATS on lung met 11/18 started xeloda
chemo break, nothing seen on scans 6/19 or 8/19 Hopeful