Please keep in mind that the heart normally sits in the center of the chest. Pains on the left side, under or within the breast, are essentially never from the heart. You don't say what the pains feel like, how long they last, what brings them on, how you relieve them, whether the pain radiates to the left arm or whether this is a separate pain, how old you are, what your family history is like, etc.
Please be aware that I am unable to diagnose medical conditions online.