Well why I think that your gf doesnt not feel like you love her is because you might not be showing her you love her. You could be flirting with another girl. Or maybe you might not be treating her with respect. There are tons of reason that she might think that. What I suggest doing is writing her a letter or singing her a song. Do something that you might not do to another girl... Or if you are old enough and really in Love with her ask her the BIG QUESTION.
Maybe she is a alien.
You may not be listening or have been very distant lately. Your girlfriend may feel like she needs more attention. Maybe that's what she expected from you. If you aren't the kind to give extensive attention, sit down with your girlfriend and talk about it. Try make her see that you can't give your attention all the time. She may need more attention than you can give, and you should separate.
Maybe you are doing stuff your girlfriend thinks you are doing , if you get what I mean? Good luck :)
Most likely because you don't do the same things you used to...she feels like your beginning to just settle and she think your not putting in the same effort as in the beginning, because this is because you are beginning to get to comfortable