
She could have picked it up anywhere on her walk or out and about.
They call it kennel cough as it is notorious for spreading in boarding kennels if one dog come in with it. If you have any other dogs they too could get it.
It is very contagous and you should keep her away for others for at least a couple of weeks. It rarely causes complications unless the dog is very young or old.
Get her some childrens Benylin from the chemist, that will help sooth it, use the childrens dose on her.