When you have Fibromyalgia, a good thing to do is to be diagnosed with your doctor and be sure to get a second opinion. Fibromyalgia may have multiple beginnings, from muscle strain to more serious conditions such as a tumor, so it's important to figure out what is the cause of your Fibromyalgia. Core characteristics of the disorder may or may not include impairing fatigue, night sweats, and tendon stiffness.