Ali,
It sounds like she knows you are inclined to think along those lines. She sounds maybe a little insecure about things too? I have often wondered when people tell there SO about there secret feelings, what the best approach is. I told my partner before we were married. Well, as I understood it then. I personally think it is a mistake to blurt out, "I am trans." That says, "I made the decision, it is done, you had no part in figuring this thing out, now what do you say?" I think it is wiser to ease into it asking for help figuring things out because I am really struggling with this gender thing. To me, you then work it out as a partnership rather than one person saying, "Like it or lump it."
Moni