Generally the left chest pain can indicate gas pain. (thats if you mean by your breast bone and not upper abdomen area) Which could be from gall bladder, GERD, even possible lung, Esophageal pain. Only thing is all of this can mimic heart problems as well. If you are unsure, it is always best to be seen to make sure this is not related to your heart. Other wise, a Gastroenterologist would be the best doctor in find more about the cause.
Here is a site you can read over as well with chest pain, if you have any of the other symptoms then do be seen. read here
I don't know which type of doctors you have seen. However I would follow up with a Gastroenterologist at this point, as you may even need to have an endoscopy done to find out what is going on. An X ray can't give much detail to what is going on.
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