My cat was found on a porch, near death from starvation. She was a little over 1 year old. Now, she has been in a loving indoor environment for about 3-4 years, and is growing like a pumpkin around her middle. She eats like a bird, but her stomach is huge and her legs and head are normal sized (aside from possible starvation-induced joint issues and "bulldog" front legs). Her coat is shiny and soft, with no skin issues.
She also does a strange "dance" with her hind legs and tail, almost as if she is in heat. She has been spayed, but she lifts up one hind leg at a time, and her butt and tail shakes and vibrates. It distracts her and seems to annoy her, but she can't seem to stop it.
My guess is your cat is on a processed pet food an more then likely contains grains and vegetables that are not species specific for the carnivore that she is. Are you also free feeding her? Leaving the food out 24/7? These woud both be causes for her being so large around the middle.
The dance you describe is normal behaviour for cats, it does not annoy her, she is doing it naturally/by instinct and is most likely purring while she is doing so. :-)
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