I Had A Panic Attack Back On 7/15/14 - Since

I had a panic attack back on 7/15/14 - since then I have had tightness on the left side of my chest. I have been in the hospital and they told me that I didn't have a heart attack, but the pressure keeps lightens up and when it comes back it comes back strong. I even had a mammogram and ultrasound on the left side last year and it showed nothing. What is causing this chest pressure??