1 wk bilateral flank pain, now a big mass deep in right abdomen ~2" below ribs. scoots upward if i squeeze my side. stools have been black/maroon but are occult neg. no other (new) symptoms. advice?

You need some tests.: Regardless of the stools being negative for occult blood i would strongly recommend starting with a urine, cbc, metabolic panel and ultrasound of the abdomen. These would not be very expensive. Other tests after these initial ones would then lead to considerations for a colonoscopy or ct of the abdomen. You need more information.

GI blood: Black stools, if not due to upper GI bleeding can be caused by taking iron or peptobismol. Maroon colored stools are quite suspicious of blood coming from the small intestine or upper colon. An abdominal ct scan or MRI should identify the mass you describe and give the radiologist and your physician enough information to make a differential diagnosis and advise you on the next course of action.