I hate to say it, but the other morning my girl that was the only one to lay fairy eggs died unexpectedly. :( She was in molt but seemed to be doing alright, then boom dead. Whether her odd eggs were a symptom of something internally wrong with her, I don't know. Hopefully with your girl...
Unfortunately it could also be genetic, which is what I suspect is the problem with my Marans that has laid two fairy eggs. None of my other girls have ever laid a small egg like that, and their shells are super strong. The Marans, when laying well, does lay jumbo eggs, but then will suddenly...