Usually nothing to worry about. With some really old hens ive found that some try to squeeze out an egg but it has a paper thin shell. With younger hens like yours its usually just a glitch. Make sure you feed them oyster shell etc because it can sometimes mean they dont have enough calcuim.
My Chicks do that quite a lot, Stretch their necks and almost seem to "Yawn" but I found that its usually thirst. I give them some water and they usually are fine afterwards, for the rest of the day until the next when they want some water when outside foraging etc. Other than that it could be...
I recently got six chicks from a hatchery. Loved them all so much, Unfortunately, just yesterday the family dog got into the brooder early in the morning and killed two of them, before I got to the scene, The four were all alive, but one had something done to her wing. Its almost folded over her...