You definitely need a second opinion. The same thing happened to me over a year ago. A new doctor told me, after 17 years (at the time) of having UC, multiple colonoscopies all saying I had it, that I did not in fact have it. Told me to stop all my meds cold turkey. I suggested a taper, like somebody else did. He also used that test (plus the colonoscopy I had before I saw him was totaly clean). I had a severe flare when I was first diagnosed and smaller flares after that, so it was clear I had UC. He said they could have just been an infection! He was ignorant and not a very experienced doctor clearly.
I started tapering my enemas and Colazal, and went into a bad flare. It was very hard to get out of but I finally am now after over a year.
Some doctors like to just 'go by the books'. They don't think about the fact that not everybody's disease is the same. Just becuase one person's illness presents one way doesn't mean the next person's will be the same. A doctor that looks at everything in your body, not just 1 test, is the type of doctor you really want.