Although my lot have some moments, there are always reasons. Bilbo doesn't like me sitting in a certain place on the couch and will dig my arm to try to make me move so he can run over the cages :roll: Or my feet are in the wrong place. We're looking after a hamster for someone right now, and Bilbo was really grumpy the day it arrived, he's completely fine now, but we think the initial smell confused him.
So, it could be a sign of something internal or external upsetting her, but the external factors can easily be something you haven't thought of.