The pain sits at the base of your skull and across the top of your shoulders, and it arrives in the afternoon like it is on a schedule.
The schedule is the clue. If the tissue were damaged it would hurt in the morning too.
What you have is a load problem that accumulates through the day.
Here is the chain. Your head weighs around eleven pounds. Held over your rib cage, the skeleton carries it. Held forward, muscles carry it, and the further forward it goes the more they carry. What determines how far forward it goes is the position of your rib cage, and what determines the position of your rib cage is how you breathe.
Sit for hours in a slightly collapsed position and you shift to shallow upper-chest breathing, because the diaphragm cannot work well in that shape. Now the muscles at the front of your neck are assisting every breath you take, roughly twenty thousand times a day, in addition to holding your head up. By mid-afternoon they are done.
What I test
Rib expansion with my hands on your lower ribs while you breathe. I want to feel lateral expansion. If the chest rises and the ribs do not move, we have found it. Then thoracic extension, then how far your head sits in front of your ribs at rest.
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What we do
Restore the breathing mechanics first, so the neck stops assisting. Restore mid-back extension, so the head has somewhere to sit. Then strengthen the deep neck flexors and the muscles that hold your shoulder blades back, so the new position holds without you thinking about it.
Stretching your neck will feel good for twenty minutes. It does not change what your ribs are doing.