Hi Liz,
While you were gone, Thursday to Sunday, did your cats have someone watching them, or did you just leave food down with a timer?
Maybe just the stress of your being gone caused Sophia some problems. She is not a cat that would eat anything unusual is she? Like a 'pica' cat? Thread, cotton batting, rug pieces, etc.? I had one cat that ate carpet stuff from one of those cat trees years ago, and got very sick, diarrhea and vomiting.
Did the vet do an x-ray? If she still is not eating, I would want an x-ray done of her stomach, just to make sure she is not full of hairballs, or anything unusual. Even a constipated cat can seem to have diarrhea, when the fluid in the intestines is able to get through past the solid feces, and sometimes there could be a partial obstruction.
I would try some baby food, though, or if you have some really tasty tuna, try a little of that with her. If she is still not eating, though, I would call the vet back and maybe have an x-ray done.
What meds did the vet prescribe? Appetite stimulants, like Cyproheptadine or Mirtazapine?
But, maybe it is all just a case of missing you and she will be right as rain by tomorrow, hopefully. I will keep you and Sophia in my prayers. Keep us posted.
Jean and Charcoal (GA)