A 25-year-old woman presents with a 6-month history of intermittent lower abdominal pain, bloating, and altered bowel habit (alternating constipation and diarrhoea). Symptoms are often worse after eating and relieved by defecation. There are no red flag symptoms (rectal bleeding, weight loss, family history of bowel cancer). Examination is unremarkable. Blood tests including FBC, CRP, and coeliac screen are normal. What is the most likely diagnosis?