Irritable bowel syndrome (IBS) is a functional disorder of the gastro-intestinal tract. No organic damage of the GIT is present during IBS. This is why all the diagnostic tests appear negative (normal). IBS can manifest with nausea, vomiting, diarrhea, acid reflux, abdominal pain, bloating, and constipation but not with fever, and hair loss. To confirm IBS as a possible diagnosis, you should first exclude all organic disorders of the GIT (peptic ulcers, inflammations, tumors, diverticulums etc.). Diagnostic procedures that remain, in your case, are an endoscopic gastroscopy, ultrasound scan and CT-scan.