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, wind, rooster eggs are usually from a tiny piece of tissue breaking loose from the reproductive tract, or an immature ova(yolk) and the body forms an egg around it. Color can be darker than 'normal' as the pigment coating released has to cover a much smaller area so is thicker. Can happen with any age layer, but more common with older layers.