It sounds positional and I suspect spine of thoracic region...
I think chest xray / thoracic xrays / likely an MRI of thoracic region may be in order ...
I would consult orthopedic specialist next ...
I suspect spine ( degenerative disk disease ) more based on history of lower back issues
I'm not thinking pulmonary problem so much as being culprit
You need further evaluation I agree
I'm not thinking that it is ominous as some of your comments allude to...
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Anthony Bray MD