These are some of the sites out there discussing fibro, weight gain and their solution to this frustrating problem. I hope you find something that helps you.
Fibromyalgia and Weight Gain at Health Central. Lack of Sleep,
Neuroendocrine Abnormalities, Decreased Activity, Medication and Weight-loss Strategies.
Weight Gain: Why It's a Problem With Fibromyalgia, and What to Do About It ProHealth.com by Mark J. Pellegrino, MD. Discusses Decreased Metabolism, Hypoglycemia [abnormally low blood sugar]and diet.
Fibromyalgia and Weight Gain at Fibromyalgia Symptoms - fatigue and diet and exercise