@nycmom I'd like to offer some of my own experience. I have thoracic outlet syndrome which does cause chest discomfort and spasms and it has affected my ability to breathe correctly. What happens is the scalene muscles in the neck can elevate the first rib and tightness across the chest muscles can interfere with the ribs expanding properly. TOS is aggravated by slouching and poor posture. When you mentioned resting and relaxation causing your symptoms, I was wondering if your posture was contributing to the problem, and if it might be TOS. I might suggest seeking the advice of a physical therapist who is familiar with TOS. You can see a specialist MD at a university medical center that treats TOS. A lot of doctors don't understand it and miss the diagnosis, and there are different variations of it. Mayo is a good place for this. I came to Mayo because I needed spine surgery, and also had TOS which was affecting my neck. What helps TOS for treatment is myofascial release. TOS can be caused by an injury or repetitive stress and your physical build can affect it too. Here are some links about TOS and our discussion about Myofascial Release. One of these articles talks about a case similar to yours where a patient was evaluated by a cardiologist and for lung function and all of it was normal.
https://www.mayoclinic.org/diseases-conditions/thoracic-outlet-syndrome/symptoms-causes/syc-20353988
https://www.painscience.com/articles/respiration-connection.php
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3501918/
https://share.upmc.com/2016/08/thoracic-outlet-syndrome-symptoms-and-causes/
https://trainingandrehabilitation.com/how-truly-treat-thoracic-outlet-syndrome/
https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/