I HATE that feeling - I get it, too! It comes over me like a wave and, like you said, it's like someone took the life out of the muscles. No rhyme or reason why most of the time. The bad pain I can understand better, because usually it's when I've done too much. But this is different. I recently had my blood sugar tested because it sounded kind of like that, but that's not it. Some of the symptoms of Fibro and CF do overlap, but someone else will probably know more.
Diagnosed: Fibromyalgia, possible Meniere's, elevated liver enzymes, skin grafting on back, arthritis, scoliosis, lumbar disc damage, sciatica, IBS, migraine headaches, tachycardia, skin cancer surgeries on face
Meds: Nortriptyline, Clonazepam, Darvocet as needed
Multivitamins, l-lysine, probiotics, magnesium, malic acid, calcium + vit. D, vit. C
Be yourself. Everyone else is already taken.