Carcinoid syndrome was the first thing that came to my mind, but it is less likely with a negative 5-HIAA. Another consideration would be mastocytosis which is a disorder of mast cells in your body. It can certainly lead to flushing as well as variety of GI symptoms - including abdominal pain, diarrhea, nausea, vomiting, peptic ulcer disease, and gastrointestinal bleeding. If this is suspected, a bone marrow biopsy can be considered for further evaluation.
Regarding bacterial overgrowth - it also may be possible, although this is associated more with loose and frequent bowel movements. A variety of breath tests can be considered to initially evaluate for this disease. The best test is known as a jejeunal aspirate, which can be done during an endoscopy. These options can be considered if suspected by your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
Medical Weblog:
kevinmd_b