That really sucks. You're probably right about that one. Or if they want to switch, they need to approach a manager first to make sure the correct shift gets switched. It's really lame that you'd get in trouble. She should get in trouble to. it was HER shift. And SHE messed up. I would argue that if you do end up getting written up.
I hope getting back on your diet helps you out. I know I felt a little better just being back on, even if it was nice to eat what I want. Good luck!