I forget where but somewhere there is a thread/ link/ page about all the weird stuff that can be found in eggs.
In my opinion, in this particular case the hens oviduct factory wasn't working correctly and sometimes bits of egg get "jammed up" on the assembly line, and are then coated with more and more layers.
You are lucky she passed it. Sometimes that happens and they can become internal layers. I would keep a good eye on her to make sure she is laying regularly.