Thanks for answering my questions.
Because of Otis' age, without being able to see him, it's a bit tricky trying to figure out what is going on with him.
The list of 'could bes' is very, very long: seizures, underlying organ disease, pain from arthritis, ingestion of something unusual, neurologic issues..
If he is mostly back to normal, you could just keep an eye on him for right now, and then evaluate him in the morning. However, if this were my dog, I'd be more prone to having him seen at an emergency vet tonight, just so they can get some blood work done to ensure there's not something systemically going on.
Hopefully it will end up being nothing too serious, but having him seen tonight is a very valid option...just to be on the safe side.