Rotten Egg from Hen?

I went out today to get my every other day egg from my Golden Sebright hen and today there were two in her nest. I brought them inside kind of stunned about finding two eggs in one day in her nest and was going to put them in the incubator but one was pretty icky looking. Looked as though it had poo on one side of it. The other one was very nice and clean. I noticed that the dirty one was pretty light compared to a normal egg so I opened it up and WOW was it rotten smelling! Then inside was clearly rotten, everything inside was grey and STUNK (i am still trying to get the smell out of the bathroom). I threw it outside of course. My question is : Is it possible for a chicken to lay a rotten egg? All her other eggs have been perfect. I thoroughly check the nest everyday to make sure non are buried so I am certain it wasn't in there before. I just don't see how a chicken could lay a rotten egg! She is the only one that uses that nest as she and her roo are seperate from the rest of my flock as these are our breeders.

 

I guess it is possible that somehow I missed it in the nest box. I thought I did a very good job at changing out the nesting material the other day, guess not! Thanks so much for the input!