The dose you have been prescribed is the "begining" dose, I do not know about those with colitis, but this really shouldn't give you diarrhea. If it does, give it few days, all meds have some side effects, some go away in a few days up to 2 weeks. Naturally, if it is a problem call your pdoc ASAP, due to your medical condition.
It is an excellent med for those who tend to have more manic episodes vs the depressive mode.
Give it a try, it gets better.
I hope all agrees with you, but always remember to keep close in touch with the prescribing doc.
If this gives you any comfort, be assured that there are still plenty of good alternatives. Don't give up hope

((((((((((( January ))))))))))))