Perhaps your girlfriend is stressed or has some other problem that makes her change her mood. Try to find out what it is, ask her nicely and using good manners.
Try to help her if you think that her problem is something that you can solve.
If you can't do anything about it, support her and encourage her, but try not to stress her more.
If you think depression is her problem convince her to visit her GP or therapist.
Good luck!