Update

Phoebe stopped shaking and shivering overnight.
Mom thinks maybe she was just cold? (which is possible since she got a haircut on Sunday so has a lot less fur right now.)

Phoebe slept fine (on my heating pad) all night long without anymore shaking symptoms.

I did make an appointment with the vet to have her electrolytes checked on Saturday morning. If she has anymore symptoms during the week I will have her go in sooner.