We have a cat that is about 13 years old. She used to be really energetic and lively up until yesterday. It literally occurred overnight. She seems really wobbly on her legs, barely ever moves, and just lies down on the floor with her head down. She seems like she has almost no energy. She drank a little bit of water but she won't eat. Periodically she meows pitifully a few times. We let her outside yesterday and she stumbled over to a patch of mulch (about 10 yards from our house) and peed, but we had to carry her inside. We are taking her to the vet tomorrow. Does anyone know what this is?