IMO, yawning is a event that happens when snakes awaken from sleep, realigning the jaws after or before a meal and a reflexive movement when external forces are applied to the upper jaw or lower jaw. As far as the fluid in her mouth are you sure she wasn't drinking water just prior to your observing it? Was it a lot of fluid or just a bit? Certainly can be suspicious, but just as a differential. Main point. Any change to your husbandry? Did you triple check the temps? Try bumping the ambient temperature a bit. The ultimate definitive action is the herp vet but try and troubleshoot everything in your power first.