Well its good to hear that somebody else has seen it at least.

One thing I did was call the manager of the exotic pet store I frequent and he reminded me that animals, like people, have their "one-off" type of thing. Technically, she did it twice but it was twice in a row so that still kind of counts as "one-off"

I also got her out and handled her for a while and listened to her breathing. No rattling or bubbling.

I haven't seen her do it again and she seems pretty normal. I do use coconut fiber bedding and I was thinking perhaps she got some in her wind-pipe somehow. I have seen mentions of coughing in other threads on here. I don't actually see any reason why a snake couldn't cough in some manner. They probably don't have the throat control we use to make coughs and sneezes really explosive. But, when you think about it, a kind of quick abdominal spasm could accomplish almost the same thing.