Hi Lucas and welcome.
Colitis is a no joke miserable mess to have! So, a multivitamin causes you to have diarrhea or constipation? I'm a bit lost as my knowledge of colitis is the screaming diarrhea bit, not constipation, during a flare. To my knowledge, constipation is often treated with Miralax or something similar.
You will likely get more answers if you start your own thread. Just because everyone will see it then.
All the Best!