Chest related pain. Please help me out.

I'm experiencing some internal pain in chest..mostly in left side of my chest.... It happens mostly when i sit in perticular position like slight bent forward and resting my upper body on my elbow.... and when i get up i feel like paining while getting into normal posture... I'm 3D Architectural Visualizer so i need to sit for long time on chair...and work on pc... And due to such pain I'm not able to work... I used to smoke...i had Quit smoking 2mths back... I used to smoke daily 4-5 cigarettes for almost 2 years... Plus I'm feeling some kind of pain in lower left side of  rib cage... I had checked for this rib pain doctors said its muscular pain...i haven't yet did any check up for chest pain... Please help me out. Thanks.