Why did my English Bull Terrier collapse today?

She is almost 5, and is, at least I thought, in good general health. Three days ago she developed a cough. Initially I thought she might have swallowed a foreign object as she has a habit of chewing and tearing her rubber toys into small pieces and also running, quite literally, through the conifer trees in our garden most afternoons while trying to get the attention of our neighbours dog. I became particularly concerned when I noticed yesterday (Sunday) that the cough seemed to be worsening. I called our regular vet surgery this morning to schedule an appointment for later this week. However, soon after I had made the call, she collapsed in the garden and, though conscious, she was unable to get up from her side. I took her immediately to the vet's. She has been there all day and so far they have said it is either "kennel cough" or an "enlarged heart". I'm not satisfied with either explanation. Firstly, she has not been near any other dogs, other than our neighbours dog, which spends most days on the other side of our garden fence, so I'm not convinced that kennel cough is likely. Secondly, she is a fit and healthy, and fairly young, terrier, so I'm not convinced she would suddenly develop an enlarged heart without my noticing. She has not had any change in her appetite or energy levels. My question is: would an enlarged heart cause her to develop a cough and to collapse, and, additionally, is it possible her heart enlarged without any visible symptoms? Many thanks to any and all respondees