Thank you very much for the useful information Joan.
This is most likely a result of spasm of the muscles around the spine and the shoulder blade.
This happens due to many reasons including;
Abnormal posture (for example at night while sleeping)
And most important a "Pinched Nerve" in the spine. It is a condition in which there is abnormal excessive pressure over the nerves as they exit the spine, and happens due to age related vertebral degeneration, or a prolapsed intervertebral disc.
At this point, you should see your doctor for a comprehensive clinical examination discussing the possibilities i have mentioned and get a management plan chalked out that suits best to your needs.
An MRI of the spine will be the most useful test here.
Muscle relaxants (prescription drugs) will also help a great deal here.
At home following measures will be helpful.
Keep yourself adequately hydrated.
Can use ibuprofen and/or paracetamol for pain control.
Muscle relaxants like tizanidine or cyclobenzaprine.
And the best muscle relaxant is "application of heating pads".
And if the pain is severe (unbearable) you should head towards ER for urgent assessment and management.
I hope this clarifies and answers your question.
I would have been worried too if I were at your place, but let me reassure you that this is a treatable condition and carries a good prognosis with appropriate management.
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