Red Rash On Neck, Burns To Touch. What Could Be Happening?

hi, i work a very labour intensive job and by three quaters the way through the day each day i start getting a red rash on both sides of my neck , by the end of the day its really red and burns to the touch, when i wake up in the morning its more or less gone and then later that day same thing, doesnt seem to matter what kind of shirt i wair, what could be happening, its very frustrating thanks