Sarah,
I had two very similar experiences. Once with a dog and once with a cat.

It sounds like your dog probably had undiagnosed hemangiosarcoma. It is always fatal. She was fortunate that she suffered for such a short time. You made the right decision for her. My dog was fine (we thought) until she started having breathing difficulties. Within four days she could no longer eat at all and was gasping for breath. She was a rescue and I had only had her three years. It is believed that she had lung cancer. Dogs slow down gradually and the signs of illness can be so subtle until it is too late. Also, their metabolisms are faster and the disease process can be very quick. My cat had diabetes and lymphoma and was in treatment. He suddenly took a turn for the worse and the ER vet recommended euthanasia that night. I still beat myself up about both decisions, but I know that I really did the best I could with the information that I had.

You should sit down and have a long talk with your vet about what was or what may have been going on with your dog. He or she will probably be able to give you insight into your experience. If you can have your questions answered, you can begin to heal. And, believe it or not, you will be able to love another dog someday.