Hi there -
This is an interesting complaint. I don't think it's likely caused by either C-pap pressure settings or neurally mediated hypotension, and cardiac pain is almost never manifested this way. However, there is one potential cardiac-related problem that might be causing it, although if it is the case this time, it's behaving in a somewhat backward fashion: that problem would be pericarditis, an irritation or inflammation of the sac surrounding the heart, sometimes with some fluid collected inside the sac as well. This can cause sharp, stabbing chest pains that can be felt in the back or beneath the sternum, and can be aggravated by breathing and can be positional. This is the "upside down" part, as usually pericarditis pain is aggravated by lying supine and relieved by sitting upright and leaning forward. However, since this does seem at least partially positional, it's one thing to consider. You could also have an inflammation of some part of the chest wall (like pleurisy, which can cause pain very similar to pericarditis), or some sort of nerve irritation.
I'd want to rule out cardiac-related causes first, even though pericarditis is rarely very serious (unless following surgery or trauma). It (as well as pleurisy) responds well to over-the-counter non-steroidal anti-inflammatory pain meds such as Advil and Aleve, so that might be worth a try.
If this continues, the nuisance value alone would be enough to cause you to go back to see the cardiologist, and although I don't think anything serious is going on, it certainly would be worth taking a look to see if for some reason the pericardium is irritated (sometimes a passing virus can even cause this).
I hope this is helpful. Best of luck to you and please follow up here as needed and update us if anything changes.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist