I am sorry to hear about your Callie. My wife and I just lost our Rat Terrier, Muffy in a similar fashion. One night last month we went to bed and she was fine. When we woke she was paralyzed. It was so sad. She was 13 but seemed to be in good perfect health.
Like you, this has me wondering the same...
I am sorry to hear about Muffy. Callie went downhill so fast, within hours. At first we thought she had gotten into rat poison or something but she never went outside off leash. Just shocking when they go so fast.