Yeah I get a pop in my sternum area when I stretch now too. Not always, but it happens every other day and feels kinda cool.
My sternum used to kill during dips and some bench as well. I'm sure I hurt one of the tendons or something in wrestling. I was muscling some guy over onto the mat and I got a sharp pain in the sternum area. Fell over on my back and got pinned.
Just take a break and ice it. My pain eventually went away.