I do remember a sharp jabbing pain out of nowhere that nearly dropped me to the floor. It only happened a couple of times and it was gone before I could even think of pain killers. It must just be a nerve sorting itself out I think as it didn't happen again.
It will be tender there for a few months. When you move your arm back from your chest you will notice a breast bone there. That was my discomfort until the skin stretched a little and it grew a bit thicker in the skin. Rubbing bio oil into the scar can help loosen that up and you don't need much.
By all means keep up the exercises but don't over do it. Just gentle movement my physio said to me, it takes its time healing.