issues can also reflect poor magnesium status.
I have a thread here on how to maintain healthy magnesium levels in the body.

And poor EFA status. EFAs may be very low after having a baby, or babies...in females. If you did not eat enough precursor oils,
like Flax, you cannot provide EPA and DHA to the growing fetus.
So then the fetus ROBS you.
Fish oil typically is used to treat some heart arrthymias.

B12 is certainly an issue. And the baby may be low if you were low during that last pregnancy. You should get tested.
I am very skeptical of any rheumatologist whol would not understand the concept of low B12 and Fibro. In fact there are studies all over showing LOW B12 exists in the CSF (cerebrospinal fluid) of patients with Fibromyalgia.
You can do a keyword search and read them easily.

Low B12 is NOT rare. Obviously this doctor is very behind the times. In fact NIH suggests B12 supplements for all people in USA over 50! Carrying children to delivery, depletes the mother significantly, as well.