I treated a respiratory infection once on a transpecos ratsnake juvie I had. I used tetracycline given to me by a vet student (who is now a vet) and followed her dosing directions. Hopefully your vet will try to get a culture to identify what pathogens are causing the infection is (if it is actually an infection)and select and appropriate antibiotic to treat it. I think most vets just "assume" its respiratory and give a broad band antibiotic like baytril or something.
Your temps and set up sound fine, so I would be stumped too as to what may be causing this all of a sudden. Could be mouthrot, which is also an infection, but is not respiratory. It is an infection of the mouth tissues that may have been caused by an injury or something. Hopefully your vet will check it out. You can also look for signs of cheesy material around the teeth and gums, or mouth sores. It could be something as simple as a piece of aspen stuck in its mouth!!
Something else to think about is a possible shed. Sometimes a few of my corns will gape a bit right before they shed. I assume it helps them loosen the skin around their lips to assist with beginning the shed. So, if your snake was blue a few days ago and just cleared up, that could be whats going on as well. HOWEVER, a vet visit never hurts.
Its a nice looking cal king and I wish you luck with its recovery.