I agree with what everyone said
Consult a new GI for another opinion. You should also get stool tests to check for bacterial sources. You would benefit from another colonoscopy. It doesn't sound like they were very accurate in describing your condition. How can it "seem like" it is confined to the rectum? either it is or it isn't. If it is just in your rectum, it should be treatable with rectal meds. Since it is not, it leads me to believe that it may extend further up. If the meds you're on aren't working you may want to consider immunosuppresents or biologics but first make sure it is not caused by something bacterial.
Did your condition begin mild and get worse? Was it always pretty much the same? I ask because I began with mild proctitis (colitis in the rectum) and it spread. After it spread, my symptoms became completely unmanagable. Luckily I have been able to control them with 5ASAs.
I think a lot of us have joint pain. It is common with UC and Chrons. I get canker sores once in a while too, I wouldn't think much of it unless I had multiple and they were happening often.
You could try a diet. The specific carbohydrate diet seems to have helped a lot of people. It did nothing for me personally.
Maybe you could increase the dose of 5ASAs? I take 3pills 3x a day.
Diagnosed with mild proctitis in March 2007: Treated with Canasa 2x Daily