A 35-year-old woman presents to her primary care physician complaining of abdominal pain, bloating, and alternating constipation and diarrhea for the past 6 months. She reports that her symptoms are often triggered by stress and occur primarily in the evening. She says that the symptoms get better with defecation. The patient reports no blood in her stools and no fever. Her medical history is significant for anxiety and depression for which she is currently being treated with an SSRI. On physical examination, her abdomen is diffusely tender to palpation but non-distended, and there are no masses or hepatosplenomegaly. A stool sample is negative for occult blood. What is the most likely diagnosis?