If this sternum pain is a pain over the front of the upper chest (the area of the sternum) and at the junction of the rib bone and breastbone, it can be due to condition known as costochondritis (is an inflammation of the cartilage that connects a rib to the breastbone).
The pain is usually sharp, but, can be mild, too, depending on the intensity of inflammation.
When there is a swelling, the condition is suspected for Tietze's Syndrome (the pain is more intensive than in your case).
Visit your physician for real diagnosis!
