Once you have done as Spotted Crow has suggested and leave the bird in a warm area sitting on a warm towel (I heat mine in the microwave)
See if she improves, if she doesn't then she may have something else going wrong with her
It may be what they call Internal laying, if the yolks can't be absorbed by the hens then it turns into EYP, which stands for Egg Yolk Peritonitis, some forms are deadly, while others the birds can live with for quite some time, even though the area your referring to does stay large and bulbish, nothing you can really do about it to be honest
Some say you can reduce the swelling by injecting and removing the liquid, but its very invasive and stressful for the bird and you and its not long term
Or your bird may have LL... which stands for Lymphoid Leukosis, its a form of cancer in that region of the birds, nothing you can do for her if it is this either, just keep them as comfortable as possible and as clean and happy as you can for the time they have left
A friends of mine has an old girl in her flock with this at the moment, she has been like it for nearly 12 months now, some days she looks like she wants to give up, then the next day she is out and about doing what all the others are doing, and apart from her swelling she looks ok for an old girl of around 9 years old