Hi and thanks for the query, severe flank pain with the symptoms described deserve an urgent evaluation. Pain, difficulty in passing gas and constipation all the same might be an indication of an intestinal obstruction. Presence of fever or not and nausea or vomiting might also need to be precised. It could be necessary to know if there is abdominal distention or not. These parameters could guide to know if its an upper or lower obstruction in case it really is. A complete blood count is required to exclude a possible acute abdomen, with a paralytic lieus. common pathologies implicated include acute appendicitis, sigmoiditis, renal coliques, peritonitis, hepatitis, cholecystitis. An abdominal ultrasound and a plain film of the abdomen is necessary to make a proper diagnosis. Complete blood counts and C reactive proteins shall be necessary to rule out an infectious process. I suggest you consult your general surgeon for evaluation. thanks and best regards, Luchuo, MD.