Well then here's my offering of a theory...
Some cats who have been exposed to cat flu during their lives and recovered from it can sometimes remain carriers of that particular strain of the virus for the rest of their lives. The way we define flu seems to differ a lot but to me it's any kind of infectious virus affecting the sinus and respiratory systems. Some people do class the sniffles as a cold though...
During times of change in their lives or stress these cats can 'shed' the flu virus, which has been laying dormant in their blood, and can again break out in flu type symptoms. A flu carrier might never shed the virus again, or they may shed it frequently. There's no rhyme or reason to it. Usually when a cat sheds a virus something daft and random can be the cause and you may not even be aware of it... although it would have helped to confirm my thinking if you'd said you've just got a new puppy or something.
Vaccination can't protect them against ALL strains of cat flu and manufacturers of vaccines tend to focus on the more serious/more frequently occurring strains. So even a cat which has been vaccinated can still catch some strains of flu. Having said that your other puss may well be protected against this specific strain via the vaccine.
So my vote is that Peanut has a touch of flu which
may require antibiotics to clear up, or she may recover on her own. Without knowing her history that would be my guess.