This might be a little far fetched but how about the nasty flu.
Last year my younger dane Zeus got back from the barn with me and an hour later he was burning up and collapsed. He was drooling and yelping and I thought he got into something. We took him to the emergency and he had blood panels done, toxin screen everything and they found nothing. They pumped him full of fluids and sent him home. He stayed like that for a few days and slowly gt better. Turns out his symptoms matched the nasty 7-10 day flu. The one where it feels like everything hurts and you can't move at all.