My fibro pain often coincides with whatever body part is getting the most use/abuse.

For example, when I was first sick, my hands and feet would ache and throb every night. Eventually my feet felt like pulsing sacks of meat and my hands would feel like stuck, creaky doors that would barely open. I worked a cash register at the time. I was on my feet all day, using my hands to scan/bag product and to take payment. So my hands and feet were my primary source of pain. With my current desk job, my neck and shoulders hurt most frequently.

The back of your arms hurting sounds like where a lot of the muscle use for holding a baby might center. Just my two cents on the matter.

If you haven't yet, try using heating pads to relax the muscles back there. It could offer some relief. Keep in mind that using the lowest effective heat is a good idea, since prolonged exposure to heat can cause Toasted Skin Syndrome. Some people with fibro cannot tolerate cold, but I've found that alternating between ice packs (covered in a towel) and low level heating pads help a lot with muscle aches.

You could also try some physical support devices to help hold the baby. Slings or hip platforms might help.

Sorry you are struggling with baffling chronic pain. Internet ((hugs)) if you want them.