An ulcer is caused by Helicobacter Pylori but you can also have H Pylori without an ulcer. It might be that you have H Pylori causing this problem, did they test for it on the upper endoscopy.
You could also ask about median arcuate ligament syndrome.
Well, without a detailed clinical evaluation it would be difficult to determine the cause of your symptoms. Possibilities that may need to be considered include GI infections, inflammations, GERD, chest wall inflammation, issues related to the respiratory tract etc. I would suggest considering an initial evaluation by a primary care physician and depending on the cause diagnosed/ suspected, it can be managed accordingly or specialist acre may be sought.
Hope this is helpful.