A 30-year-old woman presents with a 2-year history of recurrent episodes of abdominal pain, bloating, and altered bowel habit (alternating diarrhoea and constipation). The pain is typically relieved by defecation. Symptoms are often worse during stressful periods. She has no red flag symptoms (rectal bleeding, weight loss, family history of bowel cancer). Physical examination and baseline blood tests (FBC, ESR/CRP, coeliac serology) are normal. According to NICE CG61, what is the most likely diagnosis?