Is she interested in someone else?

I've been steady with my girlfriend for over two years now. There was talk of marriage and everything. Well about 4 months ago everything changed. Very noticeable to me and when I ask if everything was alright or even just tried to talk to her she would get very defensive and say that I'm being paranoid. As time went it's gotten progressively worse and she swears there's nobody else but now is to the point that she admits she sometimes really has no feelings for me or anyone for that matter. She seems to be sincere in what she's saying. Says she doesn't want to not care or feel this way but can't help it. That it's just how she feels. She is 41, isn't that a little young for like menopause or some sort of midlife crisis? We used to be great together and now I can't even get a return phone call. Through all of this, she will not break up competely. Maybe there's someone else but if not what else could be going on?