I have the same problems when my gallbladder acts up, upper back pain, tight stomach and also pain in the shoulders and collar bones.
Drink a glass of water with dissolved 1 tbs Epsom Salts , if it is the gallbladder the pain will subside withing 20 minutes, the same with the acid reflux.
The taste is not great but it is better than suffering.
If that is the case then go to the Liverflush forum and read about the cleanses at the top of the page.