Horses have a very delicate gastrointestinal system and any disturbance to a normal feeding pattern or stressful situations can result in colic or other gastric problems. These conditions are usually a result of poor management, including diet, exercise and stress. Colic is a medical emergency, so if you think your horse may have colic, you should contact your vet immediately. Common signs include pawing or scraping the ground, turning to look at the abdomen, kicking or biting the abdomen, stretching out as if needing to urinate, general restlessness (getting up and down frequently), rolling, sweating, and increased respiratory and/or heart rate.
Most gastrointestinal conditions are treatable, or at least manageable, and your horse will make a full recovery - but a full recovery depends on initial first aid treatment and prompt diagnosis and treatment from your vet, so if your horse looks under the weather, call your vet immediately.