It looks to me like she may have been vent pecked by the other chickens, and it has caused a bit of damage. She could have had a prolapsed vent where there is some of the inner tissue from the cloaca sticking out. Since that is red, the others may have pecked at her vent. They will really keep pecking at her, so yoj need to cage her or separate her. If you think that she has prolapsed, you can apply some honey or hemorrhoid cream to the vent, and it may help any swelling. Prolapsed tissue needs to be pushed back inside, but I don't see a prolapse at this time. Keep the vent area clean, and if no prolapse apply plain antibiotic ointment twice a day. BluKote spray can be applied later to cover up the red skin when you place her back in the flock.
Another thought is that she could have vent gleet, a fungal or bacterial knfection of the GI tract that causes diarrhea and redness with whitish patches around the vent, similar to a yeast infection. Antifungal creams applied to the vent area, along with oral antifungal medicines, such as nystatin may help. It does look like the other chickens pecked her also.
