Since she's in the barn make sure it is well ventilated with fresh clean bedding...you don't want dust or urine smell to irritate her lungs. If it's not windy outside I would put her out in a nice sunny lot personally...the fresh air will be good for her...if its cool out put a blanket on.
I assume you had the vet out to get a diagnosis, since she is on antibiotics, maybe call and ask them. I had a vet leave something for one of my horses who had a hay dust allergy cough, but I never used it and can't remember the name, there is something that definitely helps calm the cough, but as I said I can't remember what and it definitely came from the vet. I'm sure they could get you something, unless they feel the coughing is productive for the healing or it might interfere with the antibiotics.
There is also a product called 'respi-free' available at mose horse tack/supply locations. It is all natural/herbal and can also help provided the problem is not chronic. I would consult your vet first to ensure it is not the beginning of heaves or something more serious. Also I would be careful with ventipulman.....if you administer vent to you horse and then ride them in a dusty environment (indoor arena with other horses) you are only opening up your horses airways to welcome the dust to enter directly into your horses airways and lungs...i'd only administer vent if your horse is in a non dusty envir...or being ridden outdoors. Further do not administer liquid vent by pumping it into a dusty feed such as grain or ground flax as this also will only open up your horses airways to directly suck the dust from the feed into his/her airway/lungs
Hope this helps.