Is it really possible for a 13 week old hen to lay an egg?? I mean, I'm holding the egg in my hand so I know it's possible, but isn't this really really early??
That hen looks nowhere close to being mature enough to lay. Are you sure it was her egg? Do you had any other hens that could have gotten into her enclosure and layed? Also do you have a picture of the egg- is it full sized?