If your vet doesn't know what the cause is then you need to take her to another vet who can figure it out. Prolapses can be dangerous because if there is an injury to the prolapse the animal can end up bleeding very seriously. Anything from INSIDE the body that ends up on the OUTSIDE of the body unintentionally needs to be addressed IMMEDIATELY.
If you have an emergency vet near you I would take her there tonight. If not, then first thing tomorrow morning I would be on the phone calling some of the larger vet hospitals in your area to get her an appointment. It doesn't make sense to me that a vet doesn't know where an obvious piece of the reproductive tract is coming from, and it further bothers me that they apparently don't care enough to keep her there to try to figure it out. I would be looking for a new vet if I were you, and quickly.
Ghilly