First things first. Stop treating your cat like a dog, do not give it 3 meals a day, it only needs 2 (and plenty of fresh water). Cats need meat in their diet, no more oatmeal.

The cat may not like going outside because it might smell other cats whose territory it is.

Cats can sleep up to 18 hours a day.

If you cat is happy for 2 meat meals a day, is not losing weight and generally seems active I would not worry, otherwise if you still worry, take it to the vets