Snickers seems to have made a full recovery! We took her to the vet on Tuesday afternoon and she was already perkier than she had been for a day and a half. The vet gave her a once-over and didn't find anything unusual. So, she took blood and we got the results Wednesday night. Everything looked fine, except a few abnormalities. Her liver enzymes (maybe?) were off, but the vet said that can be a result of old age and if anything they were good for a dog her age. And her white blood cell count was up, but if she had some sort of infection or illness, that could account for that. The thing the vet found most alarming (other than our story of her behavior) is that she had lost five pounds. And really, by Wednesday evening, she was more or less back to normal. She now gets some wet dog food (which she LOVES) to help put on a couple pounds again. She's always been svelte and is now looking positively skinny.
So we think that she just got sick and had a much harder time fighting it off than she would have a few years ago because she's older. We'll take her in to get weighed in about three weeks. Other than that, things are fine as far as the canine member of our household. Yippee!