What happened to this woman?

Sorry it's been so long...I've had LOTS on my mind. Ahhh..we'll start with this.

My mom and I.

We have a really close relationship..closer than most. We're not just mom and daughter, but best friends. However, lately we've been having some issues.

See it's always been just me and my mom, so that's all I know. I mean she's dated 4 guys, including my dad (they split when I was 3), my whole life. My point is that it's always been just us. She's always been an independent, strong woman that always made her feelings known how much she despises men. She's always told me that she's happy without a man in her life. No one to control her, to tell her what to do. She can do what she wants, when she wants. And with this, she's formed how I view men and relationships too. I've always been the independent one who didn't take any kind of shit from any guy, which I love. I hate it when I see girls fawn over guys that don't call them every hour to "check in" or tell them every detail of their life or even spend every waking moment with them. Ugh. Gag me. I like being my own person, having my own time and my own friends. Thanks.
To my next point..she's been dating this one guy, we'll call him Joe. She's known Joe since she was with my dad. Joe and his wife were a mutual friends. He's a cool guy and makes my mom happy, which is what I could ever want. And I'm happy for her, honestly I am. BUT, I feel as if she's changing into someone I really don't know anymore. She's at his house or he's here 5-6 days out of the week. I understand she has her own life and she's going to live it how she wants to, but how can I go from knowing her as one person to now realizing she's changing into something different? I guess I wouldn't be worried so much about it, if it wasn't turing out to be somewhat a repeative fo her last boyfriend. They started out great and spent all their time together and he ended up moving in within 3 months. She was letting him walk all over her, taking advantage of her, and in the end she couldn't stand him. I just feel as if this is happening again. I don't want that for her. I guess that's why this whole thing is getting to me. I love my mom and I don't want that to happen for her again. I know she really likes this guy and he seems great, and I don't want to be like this. But how can I not? What does a daughter do, without nagging and seeming like the mother?