I find things like that sometimes. It is likely a bit of the reproductive tract that got shed and ended up in the egg.
The three main triggers here are
Heat, yup when it gets super hot they get stressed and weird things happen.
Environmental stress, a predator hanging about is major stress. I have body checked eggs and small bloody bits in the eggs when we have hawks hanging around.
Age can also play a part. The very young and the old hens tend to be more likely to have this issue.
I have read that a parasite load can also trigger these issues. I have not ever found a worm in the poo so I cannot say if it does or does not.
I would keep a watch on them for any odd behavior just in case there is some medical issue.
When I have found these sorts of things it has been a one time thing and then a long time with no issues.
Crossing my fingers that this was a fluke for your gal.