Pain in shoulder blade when i look down

I posted about my headaches also but yesterday afternoon i started feeling abit sore in my shoulder blade and noticed whenever i bow my head or look down it really is painful,
i don't know where its come from? my mum suggested it might be my desk chair?i only started my new job on Monday and it started happening yesterday, but ive never had it before.
My neck itself is not sore/stuff i can move it all ways no problem, its the pulling/pain in my left shoulder blade that is causing me grief.