Thalassemia Intermedia and New diagnoses of Ulcerative Colitis

my 5.5 year old was born with thalassemia intermedia.... all her levels have been fine so far... HGB doesnt drop below 9.
January 2019, we began seeing blood in stool. initially we thought she was just straining too hard. it continued on /off until we saw black stool with mucous.
after all stool studies, lab work, and endoscopy/colonoscopy later GI doc says she has ulcerative colitis.
she doesnt have diarrhea at all. it's always been lose stools or normal stools.
we are at a loss. we are definitely seeking 2nd opinion to see if she is intolerant to certain foods that causes the inflammation.

in her case she was on a low iron diet, so no white bread,pasta. we taught her to eat healthy and now she has to be low fiber, low residue foods.. this is IMPOSSIBLE for a 5.5 year old

I would definitely suggest a second opinion be found. I don't think a low fiber diet would be good for her overall health.
Common causes of blood in stools of children.
https://www.childrenscolorado.org/conditions-and-advice/conditions-and-symptoms/symptoms/stools-blood-in/

Causes of Blood in Stool

Anal Fissure. If no diarrhea, most of these children have a small tear in the anus. This is called an anal fissure. Anal fissures usually are caused by passage of a large or hard stool. This is the cause in 90% of children with blood in the stools.
Strep Skin Infection. A Strep skin infection around the anus can also cause blood-streaked stools.
Bacterial Diarrhea. If also has bloody diarrhea, a gut bacterial infection may be the cause. Examples are Shigella, Salmonella, E.Coli 0157 or Campylobacter.
Cow's Milk Colitis. Starts within the first 2 months of life. Causes loose, slimy stools. Can be blood-streaked. Treatment: need to avoid cow's milk formulas.
Blood spreads rapidly and widely in water. Passing a stool with a few blood streaks often turns the toilet water pink. It doesn't mean a large blood loss.