i think, most woman or literally girls are not the same. it depends, but i have some suggestions for you.

* she's always giving excuses to somehow avoid u, emotionally, and physically.

* she still look the same as always, but her sudden attitude change lasts too long that u can't bare 

* you might want to ask ur gf's friends about her sudden change. they might know sthg, n if ur good enough, they might tell u sthg/everything.