Can issues with bone marrow or nervous system cause widespread severe bone/joint pain morning stiffness? Arthritis ruled out

It can. But if your pain is severe, bone marrow issues that lead to pain like this isn't subtle. Alterations in your nervous system, either peripheral and/or central, can lead to diffuse pain but it doesn't tend to focal around joints. Realize other infectious and non-infectious inflammatory issues can cause diffuse pain as can mood/anxiety issues.
Generalized Pain. Pain that is not well localized may be generalized through the musculoskeletal system may be from many causes. Inflammation is nonspecific and can effect bones, muscles, joints, nerves and other organ systems. There are many causes for inflammation, and a methodical review of your medical history, a physical examination and other tests may be necessary to narrow down the cause.
Broad question. Bone marrow problems/tumors can cause bone pain but not AM stiffness.AM stiffness is more likely due to a number of other factors such as widespread inflammation (autoimmune or sensitivities to foods like sugar), lack of activity, etc. Some nerve problems can cause stiffness.A good history & phys. incl osteopathic hands-on exam may provide some answers.
Need more info. At your age considering you are not obese & have no other known musculoskeletal illness, morning stiffness is more likely a muscular problem maybe due to poor physical condition. I think it would be beneficial for you to see your family physician for a complete exam in case there is a condition you have that you are not aware of, of which there are many that can cause your complaint.

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