Chest pain when inhaling lifting weight or applying pressure to left side.possible reasons?

Muscular most likely: Most common reasons if you have no other medical conditions/risk factors are strain of the pectoralis major or intercostal muscle, and pleurisy, which is usually due to a viral infection of the lung lining. Other possibility is the start of shingles before the rash has come out; pain would be associated with tingling in this case. Costochondritis also possible.