Has she ever been the victim of rape, incest or some sort of physical/sexual violence? Sounds like maybe she suppresses it, so it probably isn't something she talks about; however the experience manifests itself during sex.

As was mentioned in an earlier response, therapy would probably help. Start off with couples therapy...then maybe she will want her own private sessions.

On the other hand, do you want to make an investment in this relationship? It's a choice you probably need to make first.