A 55-year-old woman describes a 6-month history of variable bowel habit, fluctuating between constipation and loose stools. She frequently experiences abdominal bloating and cramping pain, which is typically relieved after opening her bowels. She reports no rectal bleeding, weight loss, or family history of bowel cancer. Full blood count, ESR, CRP, and coeliac serology (TTG antibodies) are all normal. What is the most likely diagnosis?