@notacopy, here's a discussion about radiation fibrosis:
Short Course Radiation Treatment and Radiation Fibrosis
https://connect.mayoclinic.org/discussion/short-course-radiation-treatment-and-radiation-fibrosis/
See this article:
- Myalgia: Causes of muscle pain from breast cancer treatments https://www.breastcancer.org/treatment-side-effects/pain/muscle-pain

Should your diagnosis turn out to be polymyalgia rheumatica, I want to let you know there is a support group for that here too:
- Polymyalgia Rheumatica (PMR) Support Group https://connect.mayoclinic.org/group/polymyalgia-rheumatica-pmr/

I'll be interested in learning what you learn and what the rheumatologist says.