I just took my 3 month old Americauna, Louise, to the vet for this very thing. She was coughing and she just didn't look OK. Found she may have sour crop. There is a small bulge on one side of her crop that could be something stuck in there.
She needs some oxygen and some anti bio tics as we think she may have a slight infection from this.
Coughing is not good. I'm not a medical expert, but like with children, you know when something isn't right. I'd examine her and also be careful whatever it is isn't contagious.
I immediately isolated Louise from the others when I wasn't sure what was wrong. As you know, any contagious disease will spread quickly.
Geri