Its perfectly normal to want to spend time by yourself sometimes. Of course, there are a small minority of people who don't understand why someone, if given the option of spending time with someone and spending it by yourself, would choose the latter.
The way I look at it is this: its very healthy to take time to just be by yourself, so you are comfortable with yourself. I'm also sure that there are things that your GF does by herself that she doesn't consider taking time to herself, and I think that if the two of you looked, you'd probably find the same behavior in her.
