I must learn not to make radical changes to what I carry or wear when I go on long rides. Something as minor as a different backpack will change the balance of things and get magnified during the interval.
I tweaked my upper right trapezius muscles yesterday. Around mile 45 I noticed a general spasm in the back of my right shoulder. By mile 62 it was quite painful and this morning it is mightily sore. Selene massaged out the kinks and some of the spasm; the probing indicates nothing was torn.
Looking at the new pack I can now see a slight 1/2 inch asymmetry in the straps setup. This pack carries water so that added weight must have added to the problem. It's good to find these things out during training.
Meanwhile I am getting used to riding in the Seattle 'rains', which amount to no more than a soft cool shower. The Goretex rain jacket shrugged off the water keeping me warm.