Why is my shoulder so painful?

I've been working from home for the past 6 months.
I was back at work this week.
We have just a regular wheelie computer chair.
During the from my neck to my shoulder it was really painful. It felt like pins & needles & like someone was digging their sharp fingernails into my skin.
I kept doing arm stretches throughout the day & it felt like my bones were cracking.
I won't be able to get a special chair so was wondering what else can I do to stop the pain.
Why is my shoulder so painful?
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