Quiet Wheeze

[kaziopeeps]

I have 4 Rhode Island Red pullets. They are about 4 months old. Yesterday, I noticed one of them has a wheeze to her breathing. She doesn't seem to have any other symptoms - she's eating, drinking, chasing bugs, etc.
I read a little bit about some respiratory diseases chickens can get, but it didn't really help narrow down a treatment, and really only served to scare me quite a bit! I was at the feed store this morning & saw a broad spectrum antibiotic, but wasn't sure if I could safely add it to her water if she is still eating medicated feed.
I live in Virginia, and we just got through a terrible heat wave. The air temperature was above 100 in the shade for several days, with a much worse heat index. To help our girls through, we installed a small a/c window unit in the coop. When the showed signs of heat stress, my husband & I put the girls into the coop with the air on low (never super cold). We were worried about losing them due to the heat, but now I'm worried she's sick from the air conditioning.
This is all new to me, as these are my first chickens. Any advise would be appreciated. Thanks a bunch!