Harry Potter text- What does “one’s mind is going” mean?

In the last few lines of Lily’s letter to Sirius Black (The book Harry Potter), she wrote:

I don’t know how much to believe, actually, because it seems incredible that Dumbledore could ever have been friends with Gellert Grindelwald. I think her mind’s going, personally!

What does “her mind is going” mean here?

I tried to google but failed to find relevant information, is it a not-so-common phrase? If so, wonder what’s the more commonly used phrase in the similar meaning.