Hi Michael thanks for your question.
Your upper G I endoscopy
report is normal,that is good news.
Stomach has different parst and part of stomach which joins with lower end of esophagus
that is food pipe is called cardiac end commonly called as cardia,The lower end of esophagus has a sphinter which prevent gastric content and mucosa of stomach passing from stomach ino esophagus and when this sphinter is loose content of stomach and/or its mucosa pass into esophagus producing symptoms of pain and burning.There are ways to increase tone of this sphinter which you can discess with your physician.This is benign treatable condition,so nothing to worry .
Hope this answers your question.