horse breathing problem? wheezing??

Last night when we took one of the geldings out of the main herd, all the others got over excited and started running around back and forth for about 10 min. After one of the mares was wheezing, almost gasping for air. She never has done this before and I don't really know what caused it. I started her on ZEV but as it only works on the symtoms. Wondering if any one has any idea's what could have cause the wheezing. The other horses were breathing hard but not like this one mare........

If she has never exhibited this before, it might be an allergy. Is it the right time of year for that out there? It sure is here in Ontario. I had a guy who had that only when it was August. Rest of the year he was fine.

You know you could have something there. It has been the dryest year so far with very little rain. It rained last night so hopefully that will ease her weezing although the ZEV is helping that as well.