I have severe burning that comes and goes in my trapezius muscle area, solely on the left side. what can this be?

The Trapezius Trap: This is a very common site for muscle tension and pain from stress, exercise, poor posture or sitting too long at the computer (or TV, or at the microscope or as a dental hygienist, or staring for hours into a gopher hole...). Short of that, anything more serious would be way down the list, so see if you can massage-out the pain, apply heat,use Motrin and readjust your sitting &standing postures.