I experienced this for the first time yesterday. I was sitting in class and then slumped over my desk a little and when I sat up straight and took a breath, I felt pressure in the center of my chest, right in the middle of my breast bone. If I breathed in or out deeply, the pressure would intensify but when I breathed normally/shortly, there was no pressure. I thought I was maybe having a minor heart attack so of course I got nervous and my heart rate started to speed up. Then when I stood up I burped twice and the pressure went away but it's coming back this evening, but only when I inhale/exhale deeply. I'm going to see the physician as soon as I can but it's got me worried that maybe it's my heart. Have you seen the physician and if so, what did he/she tell you?
Also it's the left side and almost seems like i'm terring something if i try to breath deeper.