Where exactly in your stomach is the pain? if the endoscope was negative and they didn't actually "see" anything or all biopsy's were negative I have NO idea what it could be! Those are the most conclusive tests there are. MUCH better than the other x-ray and scan type testing. What does the pain feel like? Burning? Cramping? stabbing? I used to describe by ulcer as a charlie horse on fire in my stomach. I also have endometriosis...the early years of pain felt like I was pregnant with a porcupine. Are you male or female? Are you sure it is your "stomach" that that the pain is coming from? Sometimes pain can defer from other areas. I hope you find some answers soon!!