Without having your vet check the poop for fungus, coccidia, or worms it is hard to know the cause of her soiled vent. I would clean her up with a warm soapy bath and rinse well, towel and blow dry. Give her 1/2 tsp of Monistat or miconazole cream twice a day orally. Use the cream around the vent as well. Use probiotics or plain Greek yogurt in her diet every day for the next couple of weeks. Prevent her from getting into compost beds, and check your feed for any possible mold forming in the container.