Cats are very good at hiding pain. My first thought was that she has a dental issue. As you said, they can smell when another member is ill and they do start fights.

How old are they? Do you check their teeth, brush their teeth? Will she allow you to open her mouth to see if there is lots of plaque and tartar build up?

What do you feed? What I am getting at here is, if you feed soft food, she may well have a dental issue and still be eating alright. If you feed kibble, and a tooth issue starts, you do see them go off their feed sooner. What brand and formula do you feed them?

Oh, and of COURSE this starts on a weekend, so you have to struggle through waiting for the vet office to open!