12 weeks is a bit young to be laying.
Is her comb really red and are her pelvic bones spread?
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/
It's more likely to be a fairy egg from one of your older birds.
Open it up and see what's inside.
Fairy, fart...