What Causes Swelling In The Back Of The Neck?

I have a thickening of skin which swells up at the back of my neck. I was told by a physiotherapist this was due to bad posture. At the moment my neck hurts and I can feel pressure from my neck going up into my head and across to the back of my ears. When the pain starts in my neck I sometimes feel sick and my left arm aches and feels heavy can you tell me what could be causing this?