Help please! My dog is sick :(

I would definitely get a second opinion if it were my dog, just for my own piece of mind. It does seem like maybe your dog is depressed. Think about this for a moment when a human being goes through the trauma of surgery it can be hard knowing that you have to take it easy and not do the things you normally do. Now, imagine this from the point of view of a dog, who may not understand why he isn't feeling his normal healthy self. This could lead to the dog being depressed because he doesn't understand why he isn't feeling completely himself.