I'm a 26 year old male and i am experiencing some aching in my chest area underneath my left clavicle. it gets worse if i sit or lay down. what is it?

Depends: You need a good physical exam and better history. depending on how long and under what circumstances , and if the pain is sharp, changes with change of position. Differential diagnosis: 1)Mechanicl musculskeletal pain, worse with active or passive movement. 2) inflammatory ie pleurisy, a lung problem , infection, TB, virus, Lupus, shoulder bursitis, pericrditis 3) heart disease ie pericardial