In reply to will9118:
I have same regularly, though the root of the problem with mine is OA. Hopefully not with you but it can be essentially caused by muscle tightness of the deep muscles otherwise. Not directly caused by climbing though looking steeply up for too long esp when really concentrating belaying can over tighten the muscles in the neck which with me causes pain under the skull and base of neck.
I have to daily stretch the neck and shoulder muscle groups - walk and sit tall sensation, pull shoulders down and back slightly, massaging muscles, etc., as necessary.
Could it be postural with you? Do your trapezius muscles seem tight?
https://en.wikipedia.org/wiki/Trapezius_muscle
Best get it checked out professionally, but if it is "just" muscle tightness it is quite easy to get into the habit of releasing/relaxing them and with changes to posture (sitting/lying), minimise flair ups and the duration of them. I had to alter pillow height and density to increase support in bed, change my driving position, adjust to sitting tall, wear belay glasses, etc.
The initial muscle release may take some time - took me months to have it identified - and without consciously remembering to maintain a more relaxed posture it is easy over tighten the muscles therefore prolonging the matter and also to cause a flair up.
Post edited at 15:15