She had a seizure -- she'd had one before, at Christmas, but recovered fully. Then she was perfectly fine until she had another one last night... and I don't even want to think about what happened. It was over in less than 5 minutes; we figured out later she had passed on before we could even make it into the car.
I broke 95 on the freeway to the vet, but ultimately it didn't matter. There is nothing we could have done.
Phoebe, we miss you. :( Thank you so much for being in our lives.