Has she given birth or been carrying fry for a long time?
Intestinal or I would imagine a reproductive organ prolapse, can be associated with trying to give birth. I don't know much about it though. Prolapse could also occur with constipation.
Do you keep salt with your livebearers? What do you feed them?
A picture would help, but I might try treating with salt first and perhaps a mild antibiotic and reexamine.