Definitely sounds like a respiratory infection that is just starting. Open-mouth breathing is in the later stages, when it is harder to treat. Why would he tell you not to worry??

I really recommend you take her to the vet and get a culture done to see which antibiotics will work, and then get her started on a course. If you catch it quickly, she will recover faster and have a much higher chance of recovering.

In the meantime, up her heat AND humidity(84-85 cool side and 95-96 warm side, 70% humidity or thereabouts) and keep her in a very quiet place. Make an appointment today before it gets any worse.