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). She reports that the pain is often relieved by defecation. There are no red flag symptoms (rectal bleeding, weight loss, family history of bowel cancer). Physical examination is unremarkable. According to NICE CG61, what is the most likely diagnosis?