Anytime a cat is open mouth breathing for a while it is typically considered an emergency. Even if she is eating and doing well otherwise, it can be a sign of something bad going on. Was there any recent injuries you can think of? Your cat may just have a bit of a head cold and is unable to breathe through her nose properly, but I would to the vets as soon as possible either way. Cats are very good at hiding when they are sick or injured.
Right before Timmy died, when he started walking around with his mouth open like you are saying, he had a chest x-ray done and had a massive pleural effusion.. so they are very good at hiding things.
It may just be a stupid new silly thing she is doing for no real reason, but incase there is some kind of underlying problem it is better to get it addressed now before it progresses.