We work our horses into grass every spring. We put them out for about an hour or so for the first week, then we move them up to 2 or 3 hours, then half a day, then a work day, until they are able to be out 24 hours with lush grass. Any horse I know would colic if they have not had grass all winter then were given free range of grass. It takes about 2 weeks for us to get ours out for a full day. I just watch their poop as we progress through the week and add time. It starts to get runny then we dont move up the time. We will leave them in for the day and get it back solid. then go back a step.
Try working her into the grass instead of just kicking her out on it for a full day. Might help her and she will not colic. I have only had a horse colic on me once, and that was because they got loose and raided the feed bin.
Good Luck and I hope this helps!