A 28-year-old woman complains of recurrent episodes of abdominal bloating, cramping pain relieved by defecation, and alternating constipation and diarrhoea for the past year. Symptoms are often worse with stress. There are no red flag symptoms (weight loss, rectal bleeding, anaemia). Examination is unremarkable. Blood tests including FBC, CRP, ESR, and coeliac screen are normal. What is the most likely diagnosis?