A 24-year-old woman presents with recurrent episodes of crampy lower abdominal pain, bloating and diarrhoea over the last 6 months. There is no weight loss, rectal bleeding, nocturnal symptoms or family history of colorectal cancer or inflammatory bowel disease. Basic blood tests, including full blood count, CRP and coeliac serology, are normal. Stool calprotectin is negative. What is the most likely diagnosis?