Looking for a little advice on respiratory illness

[Boa Dave]

Good morning. 1st time poster here. Sorry to start with a question but I am concerned my Columbian Boa may have a respiratory illness. I live in Dallas NC and have called some vet clinics but have not yet scheduled an appointment. Apparently reptiles are "exotic pets" and very few vets will see them.
My boa is almost 5 years old and has lived a very healthy life thus far. She has never turned down food, gets plenty of exercise outside of her cage and is very friendly.
She turned down her fist meal a few weeks ago. I figured she was about to shed but when I held her I heard her sigh. It struck me as an odd noise I've never heard before. (I've raised boas for about 20 years)
I did some reading on respiratory diseases and found that they are normally accompanied by discharge from the nostrils, open mouth breathing, etc. She is displaying none of these symptoms. Just the occasional sigh. Still won't eat. She is very active and doesn't appear to be in poor health other than the lack of diet and occasional sigh sound.
Am I being paranoid? What should I look for? Just get her to the vet?
Her habitat is clean, warm, and the humidity is controlled. Nothing about her environment has changed.
I've attached a photo of her.
Thanks in advance for any advice.