Sharp pain in my left shoulder when used

I am 16 years old(Just to let you know i am still growing). Around 7 months ago i had sharp acute pain in my left shoulder, every time i used my left arm with any significant amount of force i would get a sharp pain in my left shoulder and it would stop me from using my left arm. I went to see a doctor and he told me that it was probably a pinched nerve ending and told me to wait it out and if the pain got worse or continued for more then a week that i should get a MRI. The pain went away after a few weeks when i stoped using my arm. I have started working out again and now the pain is coming back. It is concentrated on the top lateral portion of my upper arm.