I was diagnosed with psoriatic arthritis 15 months ago after having been diagnosed with RA for almost 3 yrs. Having said that my test never showed any elevated RA factor. The diagnosis was solely based on symptoms, much like fibro is.
Don't freak out, I know that's easier said than done. But there are tons of absolutely wonderful drugs out there to help you live a more comfortable life. Trust me, I self-inject every Monday and it makes my life bearable.
If you have never seen a Rheumatologist I would suggest it, it's very insightful.
Hang in there.