Other diseases
This point is of course very important:
If you have been found to have another condition that explains your the discomfort, then fibromyalgia syndrome has been ruled out.
Likewise, it is important to follow closely any evidence of organic disease and rule out that it could be responsible for the pain.
However, the reality is sometimes more complex. Hardly anyone has just one disease. After a certain age, for example, arthroses are added, but they usually cause local discomfort. I.e. an arthrosis does not exclude the fibromyalgia syndrome.
In addition, someone can suffer from an inflammatory rheumatic disease (e.g. rheumatoid arthritis) and at the same time from fibromyalgia. In this case, the typical joint pain gets better with antirheumatic drugs - but not the "diffuse" muscle pain and general fatigue. In this case, the diagnosis is not easy to make.
As always: you need a competent medical contact.