Cat hospitalised due to colitis. How serious are colitis cases? What tests should I ask for?

My 6 year old domestic shorthair was hospitalized yesterday due to colitis. She hasn't eaten anything since 2 days. She was also experiencing heavy blood and mucus discharge from the anus. She started showing symptoms day before yesterday - vomiting, diarrhea, lethargy, no appetite. Blood tests were done day before yesterday and were normal. X-ray was taken and they said they saw a lot of gas and poop. Just got an ultrasound done today and that's also normal. I'm at a loss for what could be causing this. I admit I didn't deworm or deflea her and her brother in time. It's been more than 5 months. She also came into contact with an outdoor cat I feed recently. But fecal tests can't be done because she's not pooping. She has also not urinated since yesterday because of stress I was told. They are waiting for her to stabilize before attempting deworming. What other tests can I ask for? Is there something else that they could do to diagnose her? I'm at a loss for what could be causing this if all tests are normal.