**
going by your case history:
1. pain while lying,on breathing, going to shoulder, improved after sitting, points towards Respiratory symptoms i.e. pleural pain or pleural pathology [ a step wise approach will help in diagnosing the condition] like
i. If there is associated fever and dyspnea:
pleuritis or
pleural effusion [which can be confirmed by reduced air entry and chest X-ray]
ii. since pain is in left side thus,
ultrasonography of liver [for
amoebic liver abscess and
cholecystitis can be excluded]
iii. if there is intercostal tenderness with diminished breath sounds, suspect empyema.
2. consult with your physician/ chest specialist for possibilities [CBC [complete blood count], Sputum Examination, X-ray chest, Thoracocentesis to differentiate transudate from exudate [if fluid is there] and treatment options, since heart [Congestive cardiac failure] can mimic the condition.
PS.
. In general take adequate Rest, nutrition and essential vitamins along with treatment recommended/given by your doctor after careful examination.