Maybe she ws spooked by something?? If it's never happened before & didn't happen again afterwards...I think it's safe to say that something spooked her.
Mimi did something similar while back that got me very worried. It went on for a couple of days...she would come running into my room (which is also her room) & hide under the bed & she would be shaking. Then one day, she stopped & never did it again.
She does shake & hide when there's thunder (even the thunder sound on the tv scares her!)...so I'm thinking some sort of sound that we couldn't hear scared her.