Why do you think its your heart? There are a lot of things in that area that can cause pain. The most common is from the esophagus, particularly where it meets the stomach. The spincter separating the 2 is located in the "cardia", part of the GI system so closely located to the heart and pain from it could therefore be conceived as heart pain. In esophageal reflux it is not working properly(It is only suppose to be allow things in, not out, unless your body needs to vomit) and often causes pain. Under the left pec is kind of a vague area description(not even sure if that means the pain is inferior or medial to the left pec). Is there a chance the shortness of breath is caused by anxiety from the chest pain? Do you take it in the morning? Most do so I just assume. Do you eat before/after and how long after, what do you eat? Also how long you have you been of Subutex and when did the pain start in relation to you taking it? How long does it last? Does moving make it worse/better?
If you are really so concerned only a hospital/Dr's office can give you any real definitive information? Nothing regarding chest pain is certain until someone does a 12 lead ECG on you, at the very least. If you are against that, possibly buy a pulse oximetry device (goes over finger tells oxygen % you blood is carrying, also will give you pulse reading)(Can be bought for around $20-100, depending on quality). If that was abnormal (<95% ) I'd go to the hospital.