Welcome to the lupus group, sorry that you're having such a hard time. Pain really is the pits I know I suffer with the stuff 24/7 as well as fatigue.
You've been given a lot of good information and I really don't have a lot add, except, that maybe you should ask your Doctor to send you to see an immunologist. I say this as I've had lupus for 26 years now, took my Doc's 13 years before the said yes and started treatment on me.
I am currently in a research study at The National Institute of Health. I have a lot issues, but my immune system isn't working properly. Of course, I've been on every single drug to treat RA/Lupus and they have suppressed my immune system big time. However, I have no B cells at all and only have 5 T cells. That's lower than your average AIDS patient with a low T cell count.
According to the Doctor at NIH, she feels that I have an Immune Deficiency Disorder in addition to my RA/Lupus and now Leukemia. I'm also possibly facing a serious issue with my lungs, which, I've already have issues with my lungs and am on oxygen 24/7. I know you are worried about your IGM and I really think it would help beneficial to see the immunologist.
Good luck with your quest to figure out what's going on. Please, feel free to ask any questions and let us know what the Doctor's decide for you.
DX- RA, Lupus, Fibro, LGL Leukemia, Pulmonary Hypertension, Bells Palsy, Gastroparsis, Blood Clots, Glaucoma, Chronic Pericarditis & Pleurisy, Severe Anemia. Way to many medications to list.