[Maria]

It's normal to be worked up for heart and lungs when complaining of chest pain. Your presenting complaint alone (you stretched and then felt a large crack and extreme pain in sternum) should give the doctor a good idea of what to look for and which tests to do to vs. heart and lung sounds/tests for chest pain of a cardiac origin.

Costocondritis (the spelling isn't correct) can mimic chest pain of a cardiac nature and is often overlooked because one is being worked up for cardiac probs. Cardiac probs can present as chest wall probs so it's common to work a person up for cardiac complaints to make sure to rule out such a problem.

Again your presenting complaint of severe or extreme pain and a loud cracking sound in your sternum upon ... (movement made) will make a difference in how you're worked up and what exam/tests might be done/ordered.

Please let us know what the doctor says/does.