Hi withit,
I think you have to apply or be getting Social Security retirement or Social Security Disability in order to get SSI because it is a supplement to the former income. You can certainly apply for SS Disability for fibro because I just did. You can also get SSD for most mental illnesses like depression and two problem seem to be better than one, so throw anything like that in too.
This site is helpful
http://www.ultimatedisabilityguide.com/index.html
The form is long, complicated and a real pain in the entire upper body, so don't try to do it all in one sitting.
Good luck!
Nan50