Cramps...
Has GOT to be the DUMBEST warning sign for someone to have when it means they're about to have to fight. Has GOT to be even DUMBER as a warning sign when it's something that's going to happen to the Slayer naturally for at least a day or two every 3-4 weeks. All those false warnings would have to get annoying as hell after a few months.
"Hello everyone. I'm Amy's imaginary friend! But I came anyway."