Can anyone tell me what causes the left front big muscle/vein on your neck to be larger than the right plus muscle tightness and slight pain on left side of neck ?

Pinched Nerve/Muscle: Most times back pain that is sudden onset will resolve. He should use anti-inflammatory meds like ibuprofen, aleve around the clock and even tylenol (acetaminophen) to help. If this does not resolve you should see your doctor for a muscle relaxant as well. If it still does not resolve, I would recommend seeing a spine specialist for a full evaluation and testing. You may need to get an MRI to see inside.