We had our little pup (Jackson) neutered this past Thursday. We picked him up Friday morning and he was fine, gorged his food down, where before he wasn't a huge eater. Yesterday, Sunday, the third day he didn' eat a thing all day. His energy seemed still there. Today he ate a little more. Should I be as concerned as I am? The vet said not too worry that much unless he becomes lethargic and weak.