Thanks for providing more information.
Possible differential diagnosis causing neck stiffness,radiating pain in arm,thumb include Nerve compression most likely in the neck but could also be at the shoulder level,Cervical spondylosis.
An MRI of the neck should be done.
You should continue physical therapy and discuss your current symptoms with your physiotherapist and if required he'll add some more exercises for pain management.
You can do massage with muscle relaxant on back of neck.
It may be possible that poor sleeping posture can be a aggravating factor.
So avoid extra pillows.
If you have associated chest pain,heaviness in chest,shortness of breath alongwith severe pain then you should visit ER.
Cardiac pathology/Ischemic heart disease should be ruled out.
Hope this helps.
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