15 Year Old Female Aussie Gagging?

I have a 15 year old blue merle Beatrice who is a rescue dog that has been the light of my life since she was 6. At 15 now she is getting a little slower, but has developed a kind of gagging reflex almost like you were dragging her on a leash.

She also seems to labor sometimes while breathing. She has been to the vet within the last year and she says her heart and lungs sound fine, it's not kennel cough or anything—but sounds like she is gagging on something. Any ideas?

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