Hello, 
Thanks for the query to H.C.M.Forum.
However there are so many reasons  for this type of pain, few of them are as followings,
1 
Pleural effusion ( fluid in left lung)
2 
Pneumonitis  ( inflammation of lung)
3 Pain of costal space,
4 Most important cause seems to be pain in stomach due  to 
gastritis, hyper acidity, over feeding. 
In your query ,you mentioned that pain relieved after defecation ( pressure released , so relief),
You also mentioned that ,when you eat something there is feeling of pain, means pain related to stomach or due to distension in stomach, pressure over lung ( left ) so feeling of pain. 
Please consult a physician and get in blood examination along 
X ray. 
Hope I have answered your  question. 
If further any question I will be happy to help. 
Wish you good recovery. 
Dr. HET