I too don't normally get any pain or ill feeling. The only side effect I get is the bloody diarrhea and feeling a bit tired.
When I was in hospital for UC the surgeon and gastro doc were recommending surgery I kept arguing that I felt fine.
We went round in circles every day with me saying "I feel fine, I'd be sitting at work right now if you'd just discharge me!"
and them saying, "you're not fine, you're very seriously ill!"
I was having ex-rays twice a day on my abdomen and blood tests four times a day checking my inflammation levels as they were very worried about
the toxic mega colon.
I have acute pan-colitis so I don't think there's any relationship between the type or severity of UC and the presence of pain. I have no idea why though!