I’m neither a doctor nor a specialist so my opinion is just that, my humble opinion. I have experienced the same tightness in the muscles of my neck. My PCP was at a loss to explain yet prescribed me muscle relaxers which basically did nothing.After many years it turned out to be Radiation Fibrosis. My doc would say my muscles were tight and I needed to relax them. Turns out he was wrong. The numbness went from my right shoulder all the way to my right foot. The docs say nerve damage.