Hi. I am 17 (although I turn 18 this sunday) . I am pretty lean and relatively muscular. For the last several months I have noticed that my heartbeat is incredibly powerful. So much so that not only can I hear it, but I can literally constantly see it pulsating lightly under my ribcage. When I exercise it becomes much more pronounced. my stomach pulsates along with my ribcage and I can hear/feel it throughout my body. Along with this pronounced heartbeat, I have had mild chest aching for quite some time. The pain is never very intense and it comes and goes when it pleases. Typically it is on my left side but it ranges anywhere from being near the sternum to being directly over my heart. occasionally I get dizzy when I stand up quickly or I will get short of breath when I excersise, but I'm not entirely sure if this is related or me just being anxious.  I went to my pediatrician, but he didn't seem very concerned (although he really made no effort to examine me)  It has been several months since then and it has not subsided at all.  Recently I was running and the pain in my ribs became significantly pronounced as I lost my breathing rhythm. afterwards I had a difficult time taking deep breaths.  However it went away after a few minutes of walking and I completed my run without an issue.  I really want to know if this is something I should be testing or if I am just being anxious.

Thank you to anyone who can help me.