My girlfriend inside and outside doesn't feel like a female?

She's your typical 17 year old blonde girl. She looks like a girl. She doesn't look masculine or anything like that. But she cries on the phone with me all the time, telling me how she hates the way she is. I asked her why. The only thing she tells me is that she DOESN'T feel like a girl. She wish she wasn't born this way because she would do anything to have been born a male. She says that she thinks like a male does; sexually. She says that it depresses her when she goes shopping for new clothes. Because she wants to dress like a guy but she doesn't want to be judged by anyone or have people call her, or think of her as a "dike". She does like girls and boys but she prefers girls much more than boys. But she tells me she would never want to turn into a trans sexual. I'm not sure if this is a disorder she might have or if she's just thinking too much about her outer appearance, but I told her that I accept her who she is, even if that means she wants to dress like a guy, just to feel happy with herself on the outside; considering she can't change her feelings on the inside. She's felt this way her whole life. She told me there hasn't been a time where she fully felt like a girl. That there's always something that makes her feel like a guy. What do you think might be wrong with her?