That's a good question indeed because most doctors will just label a person as having Fibromyalgia instead of really looking into their complaints. For instance, I was complaining of constant elbow pain. Most of the time our elbows are bent or we are resting on it to write, type, etc. Two years, almost, and no one looked into the pain. They just wrote it off as being a part of Fibromyalgia. Finally I got an xray ordered which showed calcification in my tendon from prolonged pain. I was then diagnosed with medical epicondylitis. At first, a lot of doctors pretend not to have heard of Fibromyalgia which is why it is best to go to a Rheumatologist for diagnosis. There the rheumatologist can test for other diseases that could cause joint pain or inflammation. Like many mentioned, Rheumatoid arthritis or any other forms of arthritis, lupus, thyroid issues, chronic fatigue syndrome, Lyme's disease etc.