I was diagnosed with UC four years ago, and have recently been having a flare.  I went to a new doctor for a second opinion on my treatment, and she suggested that I might have Crohns instead.  How frustrating!!  She mentioned that there are tests that can distinguish between the two - blood tests that test certain antibodies (though your body can test negative for both, like mine did, so that is not always helpful), and procedures where they actually look at your upper-digestive track.  There were two procedures she mentioned - one is called (I think) an endoscopy, where they put a tube with a camera down your throat, and the other I have forgotten the name of, but you swallow a pill with a camera in it, and it transmits pictures of your small intestine.  If you haven't already discussed these options with your doctor, it probably isn't a bad idea.  It can't hurt, right?

I hope you figure out exactly what you have, and start feeling better soon!