Lack of appetite

According to the symptoms you reported, you might be experiencing post infectious Inflammatory Bowel Disease. The symptoms of post infectious Inflammatory Bowel Disease include two or more of the following: fever, vomiting, diarrhea, or positive stool culture. The infection that caused this condition might have been too light and you did not even recognize it. The Inflammatory Bowel Disease is a functional bowel disorder and no signs can be found with most of the visualization tests you reported.
You might want to visit a gastroenterologist and have him check you for the suggested diagnose. If confirmed, he might recommend adequate therapy (antidiarrheals, serotonin-antagonists, antispasmodics).