How skinny? Cats seem really thin after they give birth. I would make sure you don't have any hidden kittens around the house. Can you check her vagina and see if there is anything different? She should have some kind of left over something on the tail and belly area, unless she's a very festidious groomer. You should be able to see if kittens have nursed on her as well. There will be wet round circles around the nipples.