Sharp pain followed by due pain

I'm 17 years old, male. 5 month ago, I had sharp pain on left chest that make me cannot take deep breath.(It happens at rest) It lasted too long for an 1-2 hours and then followed by left arm shoulder dull pain( maybe my back too). The followed pain is similar to muscular pain( same as when you use your legs too much and the muscle tired). I had EKG done but the doctor said it's normal. I find out later that it's not 100% normal. That EKG shows Sinus Arrhythmia RAD(Show on all of my EKG).

What can make me have sharp chest pain and followed by dull pain on left shoulder and arm? It happens for no reason. I thought it was Angina at first time but the doctor said that my risk to have that thing is very low because of my age. Do you have any idea? I thought about Pneumothorax too but if it was pneumothorax, the doctor must hear some strange when he listen to my chest with strethoscope.

[nonamist]

I'm really sorry about gramatical error. I'm not sure that I used correctly or not. The sharp pain comes first and then the dull pain in my left shoulder and arm come later.