A 45-year-old woman presents with a 3-month history of intermittent, colicky abdominal pain, bloating, and a change in bowel habit, alternating between diarrhoea and constipation. Symptoms are often relieved by defecation. There is no weight loss, rectal bleeding, or family history of bowel cancer. Examination is unremarkable. Blood tests including FBC, CRP, and coeliac serology are normal. What is the most likely diagnosis?