You may experience diarrhea on a frequent basis if you have ulcerative colitis. Diarrhea occurs because the digestive system attempts to remove contents of the stomach or the intestines forcefully. This often means runny, watery stool that is rapidly expelled from the rectum. Diarrhea can be acute or chronic, and it can cause a wide range of symptoms to appear, the most concerning of which is dehydration. Diarrhea can cause the body to lost large volumes of water, so it is important to continue drinking liquids. Certain diets can increase the likelihood of diarrhea.