Took Lucy to the vet. Looks like it was just a nasty flareup of her arthritis so got an increased dose of her medicine. She seems to be doing ok for the most part, and is a really happy dog normally so we're relieved that it's just something a pill can fix. She's a good dog, we'd like to keep her around as long as we can (as long as it doesn't mean she's in pain or suffering and it can't be fixed).