Did she just have a poopy butt a week ago, or could she have suffered a prolapsed vent and possibly got pecked? Flies are very opportunistic, and will lay eggs on prolapses, wounds, or a soiled vent. I hope that you can save her, but it will depend if they have gotten inside her or not. Make...
Maggots inside her vent sounds like flystrike. Can you soak her vent area in a shallow bath with soapy water, and try to remove as many maggots as possible? Use tweezers and a magnifier if needed. Are they all inside her, or are some on the outside? Can you post a picture? Maggots can kill very...