I would strongly suspect Lyme disease if severe joint pain is present and the vague "Fibromyalgia" diagnosis is involved. Lyme disease knocks down the immune system which in some people can result in Lyme induced Crohn's like symptoms.
Quick and dirty lyme test I used for my wife was giving her a natural antibiotic such as Nutrmedix Cumanda and see what the reaction is. A non infected person will not have any reaction at all. If you have Lyme you will most likely get quite ill with Flu like symptoms.
A conventional Lyme test often fails to detect the infection. That was the case for my wife. I ended up treating her myself.
You could also have an actual case of Fibromyalgia, which could involve Epstein Barr virus, Chlamydia Pneumonia, XMRV virus or some other pathogen or a variety of pathogens.
Dan