Thank you for the information,
There are some common causes of such pain on the left side.The common ones are breast abscess, mastitis, costochondritis, pleurisy, pneumonia, unstable angina, myocardial iscahemia, myocarditis, etc.
As you told that you have consulted a heart doctor and who has ruled out any cardiac cause so you should now rule out the other causes.
The mastitis of the breast tissue may also cause such pain in that region.
The other possibility can be the costochondritis which can also cause such pain in that region.The inflammation of the cartilage is associated with this condition.
You should have the relevant investigations done like blood counts, chest x-ray, exercise treadmill test, endoscopy if the symptoms are not controlled to rule out other causes.
The anti-inflammatory drugs like motrin, advil, diclofenac, nimulid, etc. will help you in the control of pain and inflammation.The antibiotics like amoxycillin, ofloxacin is needed to control the infectious cause.
Let me know if you have any doubt.