Good day, my name is Dr. G, it sounds as if your horse is having the heaves which is bascially asthma in a horse. Kinda of shocked that this happened while she was ut for pasture
Typically, heaves is a disease of horses in a dusty environment: the dust particles cause a reaction in the lungs of some horses. However, some horses can develop a similar condition in the pasture from inhaling mold spores.
Regardless, horses with heaves typically have a chronic cough, exercise intolerance, and difficulty breathing even at rest.
You cannot treat heaves with only medication: the environment must be changed. Replace hay with a complete feed and keep the barn as dust free as possible. Bronchodilator medication and cortisone are effective. Because these systemic medications can have side effects, many of these horses can be maintained by using an aerosol inhaler with a mask, just like what is used by people with asthma.
Do you have an aerosol inhaler? and a mask?, id say prob not
are you there?
I would advise to call your vet as medication and usage of an inhaler would be recommended as the next step
please let me know when you get back online so we can discuss this more!