My deepest condolences on your loss, of course you are understandably devastated to find Boy that way.
Part of the grieving process is asking why things occurred, so I hope I can help.
A severe, sudden allergic reaction is one possible cause of sudden respiratory distress and bleeding into their respiratory tract. Animals with such a severe reaction will often have hives, facial swelling, swelling of the tissues around and in the pharynx, vomit, can have diarrhea and can collapse. This would be highly unusual, but possible.
The most common cause of sudden death in cats is heart failure and collapse due to throwing a clot from undiagnosed heart disease, usually cardiomyopathy. Cardiomyopathy is a primary heart muscle disease which can be a silent killer. Cats with cardiomyopathy can appear to be normal, they will compensate for their heart disease for a period of time and when failure occurs it can be very sudden. In retrospect owners may report that their cat was slightly less active but that they assumed that was an age related sign rather then a sign of heart disease. Radiographs of the heart may look completely normal. The only good way to diagnose the disease is with an ultrasound of the heart, called an echocardiogram. Some cats with hyperthyroidism can develop secondary cardiomyopathy, often along with hypertension, which could lead to a blood clot being thrown or a bleeding episode in the brain or lungs leading to sudden death.
Another cause of sudden difficulty breathing due to pulmonary edema is biting an electrical cord. Those cats should have signs of a burn on the gums or tongue. This is much more common in kittens then adult cats.
Other rare cases of pulmonary edema and difficulty breathing are pancreatitis, systemic bacterial infections or cancer.
A previously undiagnosed, ruptured, bleeding tumor in her abdomen or chest could also cause the symptoms that you saw. We see this more in dogs then in cats though.
Please don't think that you did anything to harm your girl. I believe that she probably had undiagnosed heart disease with or without hypertension, that led to sudden death. It is horrible for you as you had no time to process her being sick, let alone her death, but please take comfort in knowing that she didn't suffer with a long painful illness and it sound like she enjoyed a wonderful life with you until the end.
Please let me know if you have any further questions.