I don't know if it's the same pain as you're talking about...But, I use to have a very sharp pain under my left breast, that went out under my arm and felt like it was going back into my shoulder blade...I was scared that it was my heart...My Dr. did an EKG and nothing was wrong with it...But he went to pushing on my diaphragm and stomach and I had "stomach ulcers" and acid reflux. I have to take Zantac every morning now...And I haven't had the pain since about a wk after taking one everyday...
I would call my Dr. and ask him about it... If when you lay down at night or after eating any acid foods, tomotoes,oranges, etc.. and you get severe heartburn and have a VERY bad discomforting feeling in your stomach it could be ulcers...My was from stress...So, I have something for that also....
Good Luck