Should I ask her what's wrong?

I think I know where she's coming from. After a wile a girl will stick a guy she's into in the "friend" category and give up on the idea that he will ever be interested back. This doesn't mean she stopped liking him. It just means she got the hint and is making steps to move on. She will still want to hang out as friends but, if she's the faithful sort, won't want to give up on the new man who genuinely is into her. She's probably feeling torn between her new love and latent feelings toward you. Just give her time and still be willing to be her friend. She'll work through it. Or, if this new relationship doesn't work out, you can be there to go out with her if you are into her. But give her time to heal first if that happens (but not too long) you don't want to be the rebound guy. If you go out with her after she's healed you won't be the rebound fix.