[patrick.n]

Well, it increased with depth, and the last time I went down I hung on the line for a while deep enough to feel it but not hurt too much, and I relaxed there, stretched the back, moved head around, rubbed neck etc trying to clear the tension or whatever was causing it. Moving the head didn't seem to make any difference.
In hindsight the second time I went in I should have duckdove, then grabbed the line to see if there's something in that initial pull from the surface that disagrees with me.
I'll be testing things out next time I get in the water.