Something's wrong

Well ask her, say something like: Listen I know that something is wrong and it's ok if you don't want to talk about it but if you do I'm here to listen, and whatever it is I'm sure we can get through it together. Just give her time she'll come around

She may just be having a bad week and it may pass. But if it really starts to affect her, sit her down and explain what you see from your point and ask if she needs your help and support.

Well it depends what is bothering her. Emotionally, physically, or anything else. If she seems different in any way sit her down, (if it's that serious) and ask her if there is something wrong.

From my experience, and assuming you have been with her for a decent amount of time...you will know when something is wrong. If you feel there is something wrong just ask. Don't hound her though, if she says things are okay wait a few days to ask again if you still think somethings up.