According to the information you provided, you might be experiencing two simultaneous conditions. The nausea, frequent burping, and middle back pain might be caused by the esophagitis, which was already diagnosed with the endoscopy. Esophagitis is an inflammation of the esophagus. It is most frequently caused by acid reflux from the stomach. There might be several causes for the reflux itself ranging from aerophagia (swallowing air when eating too fast, which consequently causes burping and acid reflux) to weak muscles on the entrance of the stomach whose function is to prevent the acid and food from returning back into esophagus.
The lower right abdominal pain and the constipation might be due to irritable bowel syndrome. This condition might be presented with either constipation and a lot of gasses trapped in the intestines, or with diarrhea. Both constipation and diarrhea might intermittently change in one person. The larger quantity of gasses trapped in the intestines stretches them and might cause feelings of dull pain deep in the abdomen. However, this diagnosis is usually given only after other conditions are ruled out. For example, lower right abdomen pain might be associated with appendicitis. Upper right abdominal pain (just below the right ribcage might be associated with cholecystitis (inflammation of the gallbladder).
You might want to visit a specialist in the upper gastrointestinal tract for the esophagitis and a specialist for the lower gastrointestinal tract for the irritable bowel syndrome. If irritable bowel syndrome is diagnosed, then simply solving the constipation and regulating the abdominal gasses might significantly relieve the pain and the discomfort.