What can be causing pain in left part of my chest when I breathe in?

Chest wall. Pain in the chest only on inspiration is usually a minor problem in the chest wall. The pleura (chest wall lining) on the inner surface beneath the ribs (you've seen this on spare ribs) may crinkle, causing a stabbing pain. Rib-cartilage joints may become inflamed "for no reason". If the pain becomes persistant, severe, or continuous, get to a physician.