Because we can talk for hours about potential reasons she wouldn't want to have sex as often as she used to but the fact is : she doesn't want to have sex as often as she used to and if you pressure her (which you do, even if you don't feel like you do), you are going to end up losing her.
But the reason is clear when I read your message: Indeed, it has nothing to do with you. I think most girls (please, other people of the said gender, correct me if I'm wrong) needs an empty head to really enjoy sex. And it seems to me that your girlfriend, currently, simply doesn't. So rather than having a lot of sex and not loving it all the time, she chooses the right time.
The variable that changed before and after is : time. Less time, less sex.
Women are really complicated as always. But just be patient and take time to talk to her and understand how she really feels about your relationship. Or let her feel that she's the most beautiful and sexiest woman in the world.