This is one of the things that can happen, you are feeling pretty much OK after surgery, they don't rx any expert PT, and many slip into the old habits or do too much too soon.
Or it could even be inner scar tissue showing up.
Some develop scar tissue quickly and some it takes years.

I would ask for some expert TOS PT , body work & massage.

Drink lots of fluids to keep the tissues hydrated.

For now do gentle range of motion and very gentle stretches and the corpse pose with deep breathing to relax.


Reading?? holding a book up?? or propped on a pillow or table?
but still the head is usually tilted or forward and can set those neck muscles off again.
our head weighs 8-12 lbs.