I've had a "mystery pain" on my left side (front) for about a year now. Ultrasound and MRI show nothing. One thing they checked me for was "ACNES", which is: Anterior Cutaneous Nerve Entrapment Syndrome, a condition characterized by chronic pain in the abdominal wall. It occurs when the anterior cutaneous nerves, which supply sensation to the abdomen, become entrapped or compressed. This was negative for me. Then they thought it could be "Mesentery Panniculitis", which is inflammation in the fatty tissue of the abdomen. This was ruled out because a lab test for inflammation in the body was negative. Just a few ideas, probably not the problem you have, but you never know. Good luck!