I went to a Physio with what I thought were shoulder issues but actually turned out to be posture issues.
When we paddled we try to get our weight forward, but some of us end up throwing our head forward too. Like this....
The effect is to put a lot of load on the C6 vertebra. If you look at the nerves that branch from that area you see that although you get pain in that area it may actually be because of the spinal alignment.
I was given some mobility exersises to do that involved pulling my chin back/in as far as I could, look up to the ceiling, then rotating my head like it was on a pole. Within a week the pain had gone and now I know the cause I make sure I keep good posture, even sitting at my PC.