Its likely not in your head, when I sat for hours at a drawing board in university it wasn't fun. I remember one night I just sort of toppled sideways because I was too stiff to move when I was done and just stayed like that for a while.

A good anti-inflammatory + a good pillow does wonders!

I'd recommend seeing a biokineticist. They're cheaper than you'd expect (at least here they are anyway). I started with mine from the sports-science dept at university. I think I was part of her thesis.

A large gym or physio practice might have one or two.

They develop a workout plan for you and walk you through it. You only go back every 8 weeks or so to adjust the plan as you get stronger. They typically have a masters degree, so they are way better than some guy who did a 2 week course to be a trainer.

Dancing or Yoga is a great start!