Coughing gagging fit

When we arrive for puppy school, Opium is rather excited (as most dogs are).
However, when she approaches the other dogs, she pulls a bit on her harness and goes into a coughing gagging fit.

Her harness is to her size, I’ve tried to loosen it off to baggy but she still had a fit today
The harness is over her chest and does not appear to be too high to hinder her neck.
Once she gets over the fit, she seems ok for the rest of the session.
I think it’s stress related but it really worries me.

I thought about using a collar instead of the harness as she doesn’t really pull (nothing compared to Nifty) but I’m not sure it would make a difference.
I will admit that we’ve not yet been able to go out on walks for various reasons, maybe this would help her ?
She does not have trouble travelling in the car.

Anyone got any ideas or thoughts ?
Thanks in advance

Is it over excitement? It sounds like it might be, in which case I would work on having her attention with you (I know she's young and that's difficult, and that you're no doubt working on it already) and see if that helps over the coming weeks. If you think she is actually suffering harm, I'd run it past your vet to make sure there is no issue with her airways.

And if she is coughing in the harness I'd definitely not switch to a collar which I can see only making it worse not better.

I think it is over excitement, well I hope it is anyways.
She only pulls once to start the fit, very odd.
I tried to rub her throat, neck and chest today but it didn’t seem to help much. It is all rather odd as she’s very uncomfortable around the other dogs, submissive and on the defensive despite being the one who will “bully” Nifty at home

Hello! The same is why. When feeling the head of the house, when other dogs "send" him as he is not accustomed to other dogs can deal with him, he is nervous about that. A greeting!