Question

ETA: It's now 18 hours later. She has been back to her normal energy and activity levels all day today. I think she is fine, although will continue to watch her. I was concerned because it was so out of character for her to be so listless yesterday. Still suspect that she may have eaten something, but time will tell there. She's always walked on leash and I pick up her poop, so I am very familiar with what's going out. ;)
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My dog, a six year old 45 pound mixed breed, has been very lethargic all day. She will not follow me around the house or go up the stairs as she normally does, but will come if called. She has just been laying around, not playing or asking to go out, although she normally does both. I coaxed her outside for her walks today and she just slowly walked (rather than being her usual leash-pulling, curious and energetic self.) Elimination is normal. She is eating and drinking normally, although not with normal enthusiasm about her meals. Her temperature is 100.8. I've made an exhaustive search of the house to try to determine if she ate something that she shouldn't have, but can't find any evidence that this has occurred. At what point should she go to the vet if she continues like this?

She is the only animal in the household and does not have contact with other dogs (she is not good with them so we avoid them). The last time she was around other dogs or cats was 9/5/2008. She does have a subluxating patella but is otherwise healthy. I don't think this has anything to do with the patella, as she is using both hind legs normally and has very specific reactions when the kneecap slips.