A fractured spleen can trigger interior bleeding that's lethal if left neglected. It will certainly trigger you extreme discomfort on the left side of your upper abdomen. Pneumonia is an infection in your lungs that can irritate your air cavities and fill them with liquid or pus. It can be moderate to dangerous. Pneumonia can bring about breast pain when you cough or breathe, which may trigger pain in either side of your upper abdomen.