Jules - a couple things here First, the colonoscopy is no problem - I had my first one at 53, and it turned into a non-event. Even the prep wasn't bad considering what us D's live with - nothing out of the ordinary. BUT...I kinda go along with the others in saying put it off a few weeks. Your wedding week should not be spent with 2 days of down time and anxiety (1 day in prep and 1 day in procedure). Leave it for later.
Second - in response to other posts here about hormones and seratonin - my attention went right up. My IBS has really been triggered by menopause - hormones. I've been following Heather's diet - though not religiously, and I really have stabilized. Of course I'm back on very low estrogen with an Estring, so that could be helping.
Third - some of your symptoms remind of something I went through a number of years ago. It turned out to be some weird intestinal thing - clostrydium difficile - or something like that. Took forever to get it diagnosed and I was ready to buy stock in Immodium before it stopped. But I was pooping straight mucous and the smell of food would have me running for the john. Major cramping and I just didn't feel very good. Anyway, you should have that checked, but no doubt you already have.
Fourth - I believe that medications can mess you up even more. Find a way to get under control without doing any prescription drugs. You have no idea what is going on if you are pumping your young body with medications. And all medications can cause other problems which only complicates the picture more.
Last - enjoy these next few weeks leading up to your wedding and have a wonderful day. You are beautiful!