Dog died of heart failure - tumour?

My Yorkshire terrier girl 15years of age passed away in the early hours of this morning(5.am). She experienced some fast short shallow breathing and occasional coughing, also a bit of sickness. We put the coughing down to kennel cough as her companion had just suffered from it. We took her to the vets within 2 hours who tested her heartbeat and chest, he told us within minutes that she was in a critical condition and might not make it through the night. He said she had fluid in the lungs and her heart was very weak which could not pump properly around the body and the fluid out...but he also said she had a tumour in her stomach which we had previously check out and been told it was a milk duct?. He was adament it was in fact a tumour.. although his main concern was the fluid on teh lungs and weakness of the heart. We are very confused at what the likelyness of death was. She had been perfectly fine in previous months and days!...eating properly, playing, barking. it was all so sudden. She had experienced minor hair loss. I understand there is nothing we can do now but if you could just give me an insight in to what you think it was down to i would be greatly appreciative. She was very happy and showed know sign of pain. We were allowed to take her home and her breathing slowed down through the night untill she gradually took her last breath.