Colic can come from a sudden switch on feed - of any kind. Colic due to change in feeds is a reaction to a change in the microbes that inhabit the gut and digest the feed. A binge on grain will cause one type of microbe to increase quickly, then when they die off, they release a toxin into the blood. With other microbes, fermentation can cause painful gas bubbles. Microbe populations can change quickly, sometimes in 24 hours.
Ditto what BareFootHooves said, feed some hay all the time, or at least a week before traveling to shows. Take enough of your hay with you to last more than the time you think you will be gone. To keep the grass digesting microbes in her system, allow her to eat small quantities of grass at the show. Hand graze a few times during the day. Even at large shows with massive parking lots, you can generally find some place to graze. Small quantities, because there may be pesticides or fertilizer on the grass.
Consider supplementing with probiotics. They aren't the end-all and be-all to avoid colic, but when changing feeds, they can really help. You would have to start these a few days before leaving, and give them until she is back on full time grass. Many probiotics came as part of a vitamin supplement. If you only want to use them for shows, find ones that are just probiotics. I've had very good results from these, but they are not available from SmartPak as some barns demand: www.equerrys.com