First off, welcome, and I'm sorry you had to find us :( This forum has got to be the #1 place on the web for getting info and support for UC. (w00t first welcome post)
Stress can cause all sorts of issues with our body. Healthy or otherwise, for some certain kinds of stress do one thing and other kinds do other things. So there is a possibility that the stress from a "good" workout might cause some issues with someone, but in reality I find it hard to believe that working out can lead to a flare.
If you enjoy a good workout but don't want to risk issues at the gym, I suggest getting you a pull up bar for at home (I have one that I can adjust for a sit up bar). I know that there is a whole post dedicated to people and their work outs here. Try to find it and you'll get some great info.
While flareing anything you can do at home, do it. You never know, working out may be theraputic for you, at least mentally if not physically.