Struggling with my driving mare

[Zelda]

I am having an issue with my driving mare. She has been conditioning since mid March. I decided a week ago I wanted to drive her at my first show. 18 days out thought it was enough time. She has driven before but its been a couple of years. Started her in the bitting rig. Great. Our weather here has been terrible but finally able to get her outside to drive. Day 1 wonderful I was so pleased she went along perfectly. Day two started this horrible breathing thing almost like she was panting while driving. Really scared me. She has never done this. She is in excellent health. Called my show friends. Said the harness was too tight. Day 2 she was better but still the same issue. After trotting around a few times started again. She isn't coughing no temp was just wormed. I am at a loss here. It doesn't seem to phase her she just keeps going along. Anyone ever experience this, any thoughts, suggestions.

 

[Marsha Cassada]

How old is she? My sister's driving horse started doing something like this. She had him scoped at the equine hospital and he had a collapsed esophagus. I hope this is not what your girl has.
The mare I acquired recently and have been working on driving is doing something that worries me, too. It doesn't sound as drastic as yours, but it concerns me. She breathes kind of noisily and acts as thought she has been running a mile when we ground drive. Not all the time, but frequently. Then yesterday, I was grooming her and she started doing it. I am wondering if it is a nervous reaction.

 

[Zelda]

She is 9 years old. I am wondering now if the throat latch is too tight. I will admit after not harnessing after a couple of years I was fumbling as to how to properly put it on. I think I am okay now. She is a very calm mare. It didn't seem to bother her ,she just kept going.

 

[Marsha Cassada]

Hope that is the easy fix!