HELP ME PAIN IN LUNGS

I started getting sharpe pains in one spot under my left nipple about 3-4 weeks ago i smoke marajuana but not cigerettes however i do mix a bit into my rizzla.  I dont drink, i am 23 year old male and the pains are sort of like a stabbing pain i suppose, i sometimes get a dull ache there too and smoking generally makes things worse... the pain has eased after using ibruprofen however i can still feel somethings there and although i have cut right down on smoking i get worried when i roll one up as sometimes this brings on the pains... I know if i stopped smoking completely would help but not sure if its the route of the problem.  i have had ECG and xray which was fine.. i am a asbestos surveyor for 3 years so cannot see this being anything to do with it due to the time (3 years) is too short for any thing to happen. but i am a bit worried doctor says pulled musscle but it feels deep and seems like  different sort os pain to what a muscle pull would.. Anywayy please help me someone :) Thanks Chris