Jeff,
it comes and goes. Sometimes it'll hit me when my head is still on the pillow in the morning. My RO warned me, pre Tx, about this issue of fibrosis in the neck muscles in the radiation field. Muscle tissue is typically stretchy, like a rubber band. Radiation makes it fibrotic and mostly non functional. Your head weighs about the same as a bowling ball and all the muscles on the minimal radiated side have to take up the slack. It's a small price for being alive...