When I first got my mare she was a little underweight and she had a dip right there. I think that is normal for that structure to show when the horse is underweight. There is a dip there naturally, it is just normally covered by muscle.
However, your girl doesn't look thin, so I'm not sure what that means. But take a look at skinny horses and their backs all have that little dip there. So why it shows on your girl, I'm not sure. But I think every horse has that dip there in their spine and it normally only shows when they are underweight.