It's sometimes normal to have some slight pain in your breasts but the slight pain usually only happens on both breasts and not just one.
When you have pain on the left breast only it's usually caused by the pleura that surrounds the left lung becoming inflamed.
The inflammation of the pleura is usually caused by an infection which is the result of the breast pain in your left breast.
If the pain continues in you left breast you really should see a doctor to get an examination of your breast to make sure nothing serious is going on.