If the cat doesn't eat or drink as much as it usually does, it starts sneezing, gets running eyes or nose or its general condition seems off the cat might be ill. Just like small children, cats can't communicate to you what's wrong with them, but you can often observe or sense that something is off.
The internet has lots of information and advice, some of it is good...and some of it is dangerously wrong. Don't trust Facebook or forums blindly. It's better to call a veterinarian one time too many than it is to skip it when you shouldn't have.