"my upper back, right on either side of the spine just where the neck hits the back, became tight and pinched"
This sounds similar to what I've experienced. It really got me occasionally in following seas. It would take quite a while to come on, and then would slowly intensify. I figured out that I could eliminate it by doing less work with my shoulders, and more with my lower torso. Pulling pretty hard with my shoulder extended too far back doing repetitive stern draws for directional control seemed to aggrevate it the most. For me, concentrating on keeping my shoulders in place and rotating from lower down is the trick. I first thought it was maybe a sore muscle, but time seemed to indicate to me that I'm susceptible right there to repetitive overuse. So I adjusted my form, not my paddling time. Much safer for my shoulders, and stronger form in the end.
I don't know if my experience is exactly the same, but it sounds pretty similar.
So I might suggest deliberately demobilizing the flex in your shoulders, even if it means shortening your stroke. The shoulder flex I'm talking about is the shoulder movement you can do without moving your opposite shoulder or the rest of your body at all - the way you can flex it back within its own socket. You could try controlling that and see if it helps.
Not claiming to be a doctor - simply sharing what sounds like a similar experience and my personal solution.
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