I experience the same! I have off and on since 2008.
When I have it, it is worsened by excessive gas or inflammation in the bowels, meaning there is pressure on my diaphragm, but it comes on before that, so I don’t think it is the cause.
I have a couple theories going on, but I’m not entirely convinced on any. One is unilateral diaphragm paralysis, I recently received x-ray results of a C4 anterolisthesis, however it was mild. Potentially it is irritating the phrenic nerve? A unilateral diaphragm paralysis can be asymptomatic, but I wonder if this is a symptom? Sometimes it is found on x-ray, sometimes hypoventilation can occur at night and it’s found on a sleep study.

Another is a maladaptive breathing pattern that started following a L shoulder car impact and wrestling injury 2 weeks apart in 2008. I don’t know the specific injury, but I know that I had sharp pain in my chest and back on the left side at the point of deep inspiration. I managed the discomfort from that injury with mainly stretching my pectorals on the left side which even now, along with my trap and other shoulder muscles seem to constantly tighten.

I did and still do have a stressful job, but 2 years ago went through a particularly traumatic time. This “breathing discomfort” (I call it, but air hunger suits it too) has come back pretty badly in the last month along with an iron deficiency (not sure how long this has been going on). I now feel I have very little reserves to manage the breathing discomfort and all the other stressors I now have in life. I found myself balling in my physiotherapist’s office yesterday.