It could be a prolapsed uterus. These gals are live bearers so it is likely a problem. Not sure how bad or what to do with this situation, and euthanizing could be needed. I may have read sometimes it works itself out, retracts back in, it was a long time back few years since I read it.
You can do some research and see. If she is swimming and doing okay maybe wait it out unless she becomes disabled or ill.
In the mean time do extra pwc
's to make sure the water is excellent and that way she can focus her strength on healing if possible.
Maybe consider adding a little salt, maybe there would be specific directions for this in your research but I would use some and also Epsom salts has healing properties. You could use the lesser dose in the thread.