Maybe she is just losing interest, but she sounds kind of depressed. Maybe you should talk to her about it. Remember that a person doesn't necessarily need something to be sad about to get depressed, a lot of times its just caused by an imbalance in your brain chemistry.
Another thing to remember after you've looked into the depression thing...
Some will, but most girls are not just going to jump on top of you and rip off your clothes. And you can't just ask your girlfriend. You really have to try to make her want it. Tease her a little and just fool around without you having the expectation of sex and just maybe you'll get it.
