I agree with Eggcessive; she looks like she has a prolapse. Heres what to do:
Isolate her immediately from the other birds. They will pick at the prolapse, and she might harm herself when running away from them, or jumping onto high rooster, etc. A dog crate would work well.
Start soaking her lower half, twice a day, in a warm water bath. Make this water 95 degrees, and soak her for twenty minutes each time. This will help relax her muscles so that she can draw the prolapse back into her body. It will also keep the prolapse moist and relatively clean.
After each soaking, and throughout the day, apply some honey and some Witch Hazel, or Preparation H. I recommend the honey and Witch Hazel, as it helped one of my birds when she had a prolapse. All three items help reduce swelling, and the honey keeps it moist and lubricated.
Once a day at least, I'd recommend trying to gently push the prolapse in. It probably won't stay in very long at a time, but should eventually help. Don't let her eat a lot of feed. If possible, only feed her at night. This prevents too much passing of droppings, which can irritate the prolapse, get it dirty, and make the prolapse protrude more. Don't restrict feed completely, and continue giving her plenty of water.
I did all of these things when one of my birds had a prolapse. Hopefully, it will help yours too, as mine successfully recovered, and has had no problems since.