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
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.
. In general take adequate Rest, nutrition and essential vitamins along with treatment recommended/given by your doctor after careful examination.