I would put on a disposable rubber glove and use some vegetable oil, mineral oil, or vaseline to lube a finger. Then stick the finger inside about an inch or so, and feel around for any hard poop and to see if the area is clear and soft. This will in turn lubricate her vent. Make sure that she is taking plenty of water, since this time of year water can become frozen, or she may be prevented from drinking normally by the other chickens if she is lower in the pecking order. Also put a little plain probiotic yogurt out for her and the others for probiotics 2-3 times a week. She could have suffered a prolapsed vent, attracting pecking by the others, and sometimes it can go back inside on it's own. Vent pecking can be something that some hens will do if they are not getting enough protein in the diet, if the coop is too small, or if they don't get outside to free range in the mornings. What are they eating? A 20% all flock feed may be good to use for awhile to bump up their protein, but also place some crushed oyster shell out for free choice feeding so they get enough calcium for their eggs.
