How can I be sure it´s fms and not something else?

Rheumatologist. ? How to know if fibromyalgia (FM) is correct diagnosis? Rheumatologists are expert at diagnosis of FM. FM is diagnosis of exclusion; laboratory & imaging studies are used to rule out other possible causes of symptoms being experienced. Diagnosis requires chronic widespread pain; including both upper & lower back regions. Depression, confused mentation, & restless sleep are common symptoms.
Get Evaluated. Get evaluated, a complete history and physical exam, in addition tests will be ordered to rule out other causes for your symptoms.
Good question. To be certain, your doctors needs to test you for other "rheumatic diseases." these are conditions that mimic fibromyalgia but can be more serious and debilitating. Finally, if it is fibromyalgia, i recommend you see an md who specializes in the treatment of the condition. You will ultimately be more likely to improve.