This is very wrong on your mother's part, very wrong. She is manipulating you, your girlfriend and your relationship and it's plain wrong. I recommend that people don't themselves tell significant others everything about their past relationships, it can cause a lot of issues, and here you have your mother telling her who knows what.
You said this is the 3rd time this has happened, did you ever say anything to your mother about it before? Did you ask her not to do it?
You absolutely have to tell your mom to back off and that what she's doing, although she may mean well, is not right. It's possible that she may not want to stop, or may not be able to stop, but if thats the case, then you will have to tackle it from the other end, which is your girlfriends end. Hopefully you can salvage your relationship with your girlfriend, but if not, you know to keep a future girlfriend away from your mom, or at least minimize their interaction until your mother shows she can respect your wishes.
I welcome your thoughts, let me know if you want to talk more