Diarrhea is a key symptom of IBS. It happens because the muscles in the gut contract more than they need to. Diarrhea may be accompanied by a feeling of muscle cramps. To produce a normal bowel movement, the gut contracts and relaxes in a rhythmic way. In IBS, however, this rhythm is disrupted. IBS can either speed up or slow down gut muscle contractions. So, IBS can cause both constipation and diarrhea at different times. As with other IBS symptoms, diarrhea is related to how the brain and gut communicate. Research into exactly why this happens is ongoing.