My Dad had the type of heart surgery where they split your sternum as they did you.

   He was sore for months afterward and he was in good shape other than his heart problem. When they split you and open you up like that a lot of cartilage and soft tissue are being moved in ways it was never meant to move.
 
It goes away in time as yours will. People would ask him what the worst part of the surgery was and he said it wasn't in the chest area it was where you're having the pain.

  I know a guy that sells replacement artificial joints to surgical centers. Part of his job is watching them put them in IF he wants too? He isn't squeamish so he watched them put in a hip socket. He told me at one point the picked up a hammer and was beating the old one to get it out, like replacing a ball joint on a motor vehicle. He asked the DR wasn't that going to hurt the patient? Dr said "that's why we dope them up good, they're always sore as hell for a while"   :o ;) ;D ;D