I'm so sorry about Jack's seizure. Poor guy... Robert, just something to keep in the back of your mind. Are you familiar with the MDR1 gene in GSDs? I was aware of it, but found out the hard way with my Sammy. He tested positive for the gene after a bad reaction to Ivermectin. Doesn't happen a lot in GSDs, but it does happen. Here's a link for you to read:
Veterinary Clinical Pharmacology Lab at the College of Veterinary Medicine
There is also a complete list of the drugs at:
www.busteralert.org
Depending on what all he was given yesterday (and I would be particiularly concerned if they gave him a large dose of Ivermectin), seizures can occur. Severe reactions occur almost immediately, but like in Sammy's case, it was about three days later that he became very unstable on his feet and very lethargic. Thankfully, Sammy experienced only subacute symptoms. He was flushed with fluids for three days and the rest of it was up to his body to come back when it could. It can take days or weeks sometimes.
Just be aware in case you start to see more severe symptoms.