Yeah, I had the same thing when I went to Mexico. It's a bacterial infection typically called "Montezuma's Revenge". My UC was in remission at the time and it brought on a flare. Even though Antibiotics are contra-indicated with UC, I was forced to take it to kick the bacteria and the green poop.
If your intestines were in a weakened state, this could trigger the onset of UC for you for the first time