Hi there, this is Dr. Elaine.
I need you to bring her inside and check under her feathers and around her vent for any apparent wound or skin irritation.
This sounds like she has been struck by maggots (fly larvae). They will often go after an animal that may have a wound or pasted feces in the area in which the fly lays the eggs.
Normally the maggots will go a bit inside the vent, but not up into the body, as they need air to breath. The ones seen to be coming out are not more than 1/2 inch or so inside usually.
Most maggots attack dying flesh, feces--so the fact that she has some sort of wound may be due to either feces pasting on her (maybe diarrhea) or she is literally being hen-pecked (or rooster-pecked). Chickens are notorious--i.e. "PECKING" order...
Hens too, are remarkably tough compared to other bird species. This area is going to require medical care. If a trip to the emergency clinic is just not possible (she really should be seen asap if these are maggots) for now, try to soak just that part of her in warm water to physically remove what you can--please wear gloves. Maggot removal can be laborious and take many attempts to get them all and is best done by a veterinarian. Good luck.
Please let me know if I may be of further help.