If you have pain with breathing in and out deaply, when sitting still, but not when you change your breathing to your normal pattern, then you are most likely not having a heart attack. How you breath shouldn't affect pain from a heart that isn't getting enough oxygen.

Pneumothorax usually affects tall people, and is most likely very painful, It reduces the oxygen that you can take into your lungs( This condition is a lung that has a whole in it. Air gets between the lung and the chest wall, collapsing the lung, which is not a good thing)

All that said, it's always wounderful to visit your doctor!