Irritable bowel syndrome (IBS) is a functional disorder of the intestines, which means that no organic disorder of the digestive tract is present in the form of tumors, inflammations, congenital anomalies…etc . An IBS Diagnosis is established when all organic diagnosis are excluded. A CT-scan is one of the diagnostic procedures used for diagnosing such disorders, especially tumors. Disorders such as inflammation, however, can’t be diagnosed with CT-scan. this is why doctors use colonoscopies with biopsy to reveal chronic inflammation.
In your case the CT-scan, ultrasound and colonoscopy didn’t show anything abnormal. Therefore, you could be experiencing IBS.