What Is The Pain And Discomfort In My Left Left Chest?

I'm 24 years old and consider myself very healthy. The left side of my chest close to my breast plate has felt uncomfortable. Not real painful just barely. But its still scaring me. How do I know if its my heart of a muscle or something? Its been like this for 7 months or so... and it doesn't hurt all the time just here and there. I play a ton of softball so it could be from swinging a bat and stretching something but idk