Dog is having Seizures then while at vet had pulmonary edema(fluid in the lungs)?

My Great swiss mountain dog, 14 months old had a batch of seizures about a month ago. 3 grand mal seizures within 12 hours. We took her to the vet where she was prescribed Diazepam orally for a month then let off to see if the siezures came back at which point we would start Phenobarbital. She seized about 36 hours after the last Diazepam. We gave her diazepam rectally during the first, 2 ampules then she had another shortly thereafter at which point we took her to the vet where she seized on the ride there and again at the vet clinic. While at the vet while they were checking her over and found fluid in her lungs. I'm wondering what might have caused this as they are saying that edema unrelated to the seizures/epilepsy and could have been a preexisting condition. She seemed perfectly normal earlier that day as we were running and playing pretty hard at the local dog park. I find it hard to believe that the dog was fine and running full out with her lungs half full of fluid. She would have had a hard time doing anything strenuous on half lung capacity. Does anyone know what else could cause this and if it could be related to the siezures? We are taking her for toxicology screening and probably for a CT scan to see if there is something else that they can find. I know epilepsy is common in this breed I just don't understand how these two things could happen so close together and not be related. Anyone ever heard of this or something similar, I'm worried for my girl.