I had the same problem with some chicks that stayed in the mail a bit too long. I dabbed a bit of neosporin on it after cleaning the poop off. It worked well and they no longer have this problem--less than a week later.
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Doing fine now. She's eating and drinking and acting alright.Her vent still looks red and plump,but she's pooping good,and its still clean around the vent.
Glad she's better! You may want to watch just to make sure no one's pecking at her vent. When they get red/inflammed, it can be sort of like a little target for the others.