I took her to a different vet last night. They saw me the day I called and were very professional and helpful. They ruled out, pretty conclusively, the idea of a blood clot, and then I did spring for an XRay which came up normal (thank heaven) and some blood work, about which I will find out tomorrow. This vet suspects diabetes or some such something. Anyway, I spent a bundle but not the moon and came home a lot more cheerful. ALSO, she's really not in pain. She doesn't feel well, but she doesn't hurt which makes us all feel Much Happier.
And for clarification, this is my big white kitty (Siamese/calico I found out last night) and not the kitty who was lost and found. I really need to go take a picture of her. She has beautiful blue eyes and she's enormously fat.