When the sternum is involved costochondritis it can be an inflammation of the cartilage connecting to your breastbone. It's a common cause of chest pain. Mild to severe pain, tenderness and soreness near the breastbone (usually the upper left side) are symptoms, and the pain gets worse if you stretch or apply pressure to the area. That should be ruled out as the cause and seems unlikely due to the "popping" sensation.
Tietze Syndrome, on the other hand, is associated with swellings at the rib-cartilage junction, and lasts longer. (Costochondritis has no noticeable swelling.) Neither condition involves pus or abscess formation. Tietze syndrome usually affects the bone-cartilage junctions of the second and third ribs. The swelling may last for several months. The syndrome can develop as a complication of surgery on your sternum, months to years after the operation. However, the cause is usually unknown.
It could be the breast bone has not completely heeled and has been aggravated from time to time and now causing the pain. Seems unlikely after so many years, but it is possible.
Tietze syndrome usually affects the bone-cartilage junctions of the second and third ribs. The swelling may last for several months. The syndrome can develop as a complication of surgery on your sternum, months to years after the operation. However, the cause is usually unknown.