What country are you in? If you have human calcium tablets give her one crushed with food. Or you can finely ground a whole egg shell and add it to some plain yogurt or a small amount of egg. She looks like she has some runny poop or some egg material on her vent area. I agree with everyone else that she may be having a problem with soft or shell-less eggs, or has had an egg break inside. Sometimes you can see an egg membrane hanging out of the vent, and the other chickens may grab it.