Why do i get random shooting pains in my heart whenever i breathe in too hard. this only occurs rarely?

Unknown: This is unlikely a cardiac problem.

Probably nothing: Probably nothing to be concerned about. Some causes are nerve pain and inflammation of the pericardium or pleura where it rubs against the breast bone ( in small built individuals) or some other unknown cause. If it is infrequent and lasts a few seconds it is generally innocuous. Still i would talk to my doctor about it.