'With' includes the object in the action. So if you say "She is being very nice with me," you are "being nice" together to some other unnamed person. In the case of talking "with" you, then you are both talking. It can mean just the two of you talking with each other. If she is talking "to" you, then it probably means that you are the audience. However, I have heard "talking to" to reference a conversation, so it is somewhat interchangeable.