Pretend you are a doctor. I'm feeling a subtle pain in my chest and my sternum feels like I need to stretch it until it pops. Am I having a heart attack? Is there anything I need to be worried about?
No, this is unlikely to be a heart attack, but you should still take care of it. I'd suggest you get up and move around a bit.

Mild pain and tightness of the sternum can be caused by spasms in the muscles surrounding the chest, back, and shoulders. Such spasms may cause the joints and other tissues in the area to shift, leading to popping of the sternum. If the popping and pain occurred with a muscle spasm, it is likely that the spasm is causing these symptoms.

Swelling, and therefore popping, of the sternum may also be caused by straining of the associated joints or muscles. While most doctors simply advise rest, it may be prudent to seek medical attention to confirm that it is a strain and not something more serious, like a fracture.