You may want to do a self - analysis before consultation with a doctor. You have had the pain since cabg that would almost rule out broken wires. If there are broken wires, the sternum would be unstable and individuals can feel a movement when coughing, deep breathing, etc. and can be associated with and infected there will be a discharge and fever.
Pericarditis may be encountered post-CABG, causing chest pain and discomfort. This is inflammation of fluid between the outer sac and heart muscle. Pleural effusion is common after CABG, more so in recipients of internal mammary grafts. The typical presenting complaints associated with pleural effusion are shortness of breath...do you have shortness of breath?
There could be nerve damage as well..
Ken
You need tests asap. It could be that one of the wires holding your sternum together initially has broken and is causing internal damage.