I am a counselor not a medical person, but wow I would be freaking out too.
Here is on option you might consider while thinking through your past. I would assume if something penetrated your back recently you would know this and I also assume you would know if you had an implant in this area that released fibers. Ruling those out, you could have had an injury a good time ago where particles penetrated your back. Be it a car accident or a childhood stick in the back.
I have seen cases where particles existed for years before working their way out. It happens so gradually the body does not respond once healed around it.
I do hope you are healing and better now that they have been removed.