Hi, use to get a bit of pain in the middle of chest a few years ago, nothing really to worry about as far as I was concerned, just now and again i use to feel a little bit of feel there.
One day I stretched my chest
(like the starting position to do a chest
fly) and I heard it "crack" meaning the bone went back into position. Try it, stretching your body with carrying in your back backwards and opening your chest should put back whatever bone is out of place in the upper region.