I'm going to maybe try to give you a perspective from her side of things... because I feel like I am your jealous girlfriend!
Ask her - has she been betrayed before? Maybe her feelings about a relationship are that if you didn't have anything to hide, she would have known about the conversation. Maybe someone in her past had an innocent chatty conversation turn into much more. It is wrong for her to bring it up over and over again, especially if there IS NO REASON for her to bring it up again! If you have been forthcoming since then and or ceased conversations with females she doesn't know, you're not hiding texts or phone calls or emails, then she should drop it! Maybe if you take extra care to show her regularly she has nothing to fear or worry about, then she will begin to settle down about it. She might just need some extra reassurance!