I get that too. It's like the center part of my chest, a few inches below my clavicle disconnects from whatever it is attached to. It happens when I stretch my arms up and lean backward. Oddly, I keep wanting to do it, but that area is becoming sore and feels like I shouldn't be doing that. My sister once separated her sternum from whatever it is attached to. i think that's what we have, but probably to a lesser degree. She wasn't allowed to move that area until it reattached, but she kept doing it anyway. I am going to stop doing that stretch. A few years ago I was doing it and the same thing happened. Then years when by with no incident because i wasn't stretching like that. I am in yoga now and it feels good to do it, but when that thing makes that gross velcro sound, and I can feel something pulling up off my chest, I remind myself not to do it again.
not so sure, but i broke my sternum quite a few years back and this happens to me. Mine can hurt as well if i stretch the wrong way. If there is no history of injury, it might be something you get checked out for and they can always do and xray. regards!!