Happens to me sometimes too if we're talking about the same thing. I've concluded that it might be a bruised intercostal above the diaphragm (or something like that) and probably involves the sternum somehow. Takes about 2 days for the pain to build up to maximum, then gets better every day until it finally vanishes after about a week. Very strange how it's so proportional to depth of breath, but pretty sure it's not a cardiac threat since there is no accompanying arrhythmia.