She's probably just sensitive and feels scared by what she sees as conflict. I was (am) like that!

Obviously you want to protect her from it as much as you can, but you also want to teach her coping skills. After all, conflict (whether perceived or actual) is a part of life. Maybe when it happens, you guys can sit down and talk with her and let her know everything is ok with mummy and daddy. I'm sure you do that already!

Good luck!