The problem you are describing can be due to muscular pain
as suggested by many doctors to you. Although according to description, you have investigations done to rule out the causes associtaed with condition of lungs and gastric condition.
In my opinion you should have also consulted to a cardiologist to rule out the possibility of any cardiac pathology which may be causing this pain. Although it is rare at this age, but it is important to rule out the cardiac pathology in chest pain
I would suggest you to consult some doctor and have investigations done to rule out the cardiac pathology for chest pain.
Thanks and take care.