How we wish it were simple to diagnose from the distance, we'd never see a doctor.
Here's a guess.
A pain or a discomfort or just an irritating niggle occurs in the morning. The body cringes, steers away from it, ignores it or pretends it will go away, but basically distorts in some way to either accomodate or try to defy it.
This is referred via some abnormal posture to weaker unused muscle or tendon and comes out as stress somewhere else.
While asleep or in lower-key afternoon/evening activity it eases away.
How to avoid it?
Conscious posture, better posture, active posture and
lots and lots of exercise, stretches, dance and activity.
If doing repetitive or other activity already, the new pattern adopted (due to the irritation) may be stressful. Something like carrying a child on your hip all day could be a factor, or playing golf for that matter.
Please ignore me if i am way off the mark.
oops, rushed through, missed Wind_rider post...