33 Year Old Female With Chest Pain On Left Side

33 year old female with chest pain on left side radiating up to collar bone at times. Was noticeable for about a week before pain became more noticeable combined with increase in resting heart rate up to 114 beats per minute normally in low 70s. Went to ER where EKG and chest X-Ray was performed and results came back fine. Blood tests ruled out blood clots, thyroid issue and the other major problems that could cause similar symptoms. Only issue found was low potassium levels not caused by vomiting, diarrhea, or fluid pill usage. Was prescribed potassium. Pain still present 4 days later. heart rate came down 12 hrs after ER visit but increases more than normal climbing stairs or with any kind of physical exertion. I was told my heart could possibly be compensating for something, but tests performed could not pin point the issue.what could the problem be?