A 25-year-old woman presents with a 3-month history of intermittent diffuse abdominal pain, bloating, and alternating bowel habits (constipation and diarrhoea). Symptoms are often worse after eating and relieved by defecation. There is no weight loss, rectal bleeding, or family history of bowel cancer or IBD. Full blood count, CRP, and coeliac serology are normal. What is the most likely diagnosis?