I think that people suffering from fibromyalgia have a successive, low-grade chronic infection. A healthy diet is essential to battling this infection, along with proper treatment.
The reason for your weight gain is probably due to a low thyroid, which the standard lab tests often fail to detect.
I hope you will check out our health pages for additional information:
Also, to find a physician in your area who frequently diagnoses and treats fibromyalgia, consider googling, "Co Cure's Good Doctor List"