I know we tend to think of all the bad and serious things that might be wrong, but in doing so we in danger of overlooking the simple ones, of which there are probably many more, and are much more common as pain instigators.
It sounds to me like a simple case of very tight muscles, as you suggest.
Tight and knotted back muscles can't relax and lengthen as they should, so they get sore, and pull down on the neck muscles. They then hurt too.
These then pull on the scalp muscles, making the head sore as well.
So you can have a variety of very strong pain in your back, shoulders, arms, neck and head, even the eyes, just from muscle problems.
I know, I have it bad.
The remedy for me is regular remedial massages. Pain went immediately. Still comes back if I overdo it, but much, much less and more easily controlled.
My new pain doctor diagnosed this problem, and he was 100% correct.

I also do a stretch and exercise routine for 30m most days.