Hi Pottens,
My mother suffers from burning mouth syndrome due to a necessary medication, and it seems like a horrible thing to deal with. I've never used a CPAP, but understand that they can cause mouth dryness. Many people with fibromyalgia suffer from dry mucous membranes - this can manifest as dry mouth (xerostomia), dry eyes, and even nose bleeds - which in turn can cause BMS. So, perhaps combined factors are compounding what you are experiencing. Dehydration is bound to make this worse, so do drink more water if you aren't already. Medications can trigger xerostomia and BMS, so perhaps review any prescriptions/supplements that you're taking with your doctor. Sicca Syndrome and Sjögren’s Syndrome could also be something to look at. If mouth dryness is indeed the cause, there are apparently mouthwashes and medications that can trigger more saliva production - perhaps that would be helpful. That's the best I can advise, not being an expert
I'd consult with a doctor or perhaps even a dentist. I really hope you get to the bottom of everything and feel better soon!