Chest discomfort

I have been having strange abnormal chest discomfort especially on the left side of my chest that i thought i had problems related to my heart or lung. I also thought i was having a heart attack. I had went to a doctor over the past 12 months period and did a checkup. The doctor listen to my heart for irregular heartbeat for signs of heart disease or pneumonia. The doctor said everything is fine and prescribed me with OTC medication for my reflex. For the past 2 months I noticed that i got dyspnea during physical exertion like running for less than 5 minutes but felt like i had ran atleast a mile non-stop. I was panting for air and my chest would feel pressured, heaviness and also with sorta pain when I breathe. Couple of days till now I experience discomfort in my chest sometimes on the right side but mostly on the left side. Reflex is under control but this left chest discomfort linger after heartburn disappear. Is this discomfort serious? How can tell from heartburn or cardiac chest pain?

I am currently 19 years old and does not have any family history of heart disease, doesn't smoke and blood pressure indicate average. There's no pain or any other symptoms in any of this, only the discomforts, which sometimes are as annoying as an itch that you just can't scratch, although I'm worried it might be something serious. Just sitting here right now I dont feel anything. For some reasons it comes and go and if it does come its relatively mild.

I also experience exhaustion, nausea, dizziness (feeling like fainting), malaise, left chest discomfort. I thought I had chronic fatigue disorder or something because I never felt this tired before. It felt like my body is tired but not my mind.