Hopefully someone can shed some light on this situation until a cardiologist appointment is done...

The dog is a 6 year old female that has never had any prior issues or symptoms etc. She is extremely active, hardly ever stops until bedtime. Current vet visit was for a UTI for which antibiotics were prescribed. Immediately after starting the meds, behavior changes, all the way to vomitting of foam occured.
This dog then was taken back to the Vet, and during a full exam, this Vet could not properly/satisfactorily hear the heartbeat. She asked permission to do an xray, where a picture of an enlarged heart appeared. So this Vet(small town vet) asks to do an EKG of which she will have to send via phoneline to her partner in NY... whom once they looked at the EKG diagnosed this dog with dilated cardiomyopathy. The onsite, examining vet does not have sonogram etc, so referred this dog's owner to another place which she has not yet gotten in for her appt...

Question is: Could this have been an allergic reaction to the meds, and could it have caused an enlarged heart? Or did the by chance exam find a pre-existing condition? Thanks for ANY insight!