For reasons related to embryology, true heart pain is felt dead center in the chest, even though the heart itself is not located there. The pain you feel behind your left breast is therefore not heart pain.
It could be something called chostochronditis, an inflammation of parts of the rib cage that is much more common in women:
It could be some weird spasms of the tiny muscles between the ribs.
The cause could also be, considering your history of anxiety and panic--no insult intended--be related to your emotional state.
In any case, since you are young, female, and have repeatedly had your heart tested, you are obviously basically healthy and the chest pains you are feeling have probably as much medical significance as those eyelid twitches that sometimes show up when people are tired or tense.
However, since you are having some scary feelings related to your heart since your diagnosis of pvcs, it would be worthwhile to talk with a professional (psychiatrist or mental health counselor) who specializes in anxiety to learn how to deal with the fear.
Doing some research on 'cardiac neurosis' and taking a look at the anxiety forum here might help, too:
but you will get the quickest relief from professional counseling.