Back and chest pain, im just 17, HELP!!?

hey! so i've been having this chest and backpains at the the left part of my body (backpain is near upper part of torso beside spine.) (chestpain is just below the left ). it starts with a sudden sharp pain, and i wasnt exercising just walking or sitting down and suddenly... the pain comes and it stays for between 5-50mins max... if i try to breathe in or move it hurts more! its been going on for years, last time it comes randomly and occasionally, like one or twice a month? now its becoming more recent, like almost everyday & more painful. i suspect it was from an accident when a heavy wooden door fell on my back... its been 6yrs thou. I've just visited the doctor he says he cant refer me to anywhere coz there isnt a doctor that specializes in only chest wall muscles... and yes, ive visited a cardiologist and he says my heart is perfectly fine... they tell me muscle pain will go away on its own but this has been going on for too long & is just worsening.. is this sth to do with my diaphragm? what can I do to get rid of this pain? ):