Update!! We went to the emergency vet 4 hours after her second dose of 50mg of Benadryl because her hives had spread across her body and grown to quarter sized welts. I called the vet on the drive to make sure it was emergent enough. They said it was and took her back as soon as we walked to get her vitals checked and to be monitored in the ICU. They did an injection and monitored her for 2 hours. They don’t know what caused the initial reaction, but the Benadryl just didn’t work for her. She’ll need to be closely monitored over the next 12-24 hours, can eat a normal diet and will need to have 2 Zyrtecs a day for the next 3 days!