Help understanding what caused dog's death

My 13 year old Yorkie-Maltese mix (spayed) died unexpectedly last weekend from some sort of respiratory distress and I am trying to figure out what caused it. She had started having a strange fast, labored breathing the Monday before, though it only happened while she was resting. Otherwise she was eating, sleeping, and getting around maybe a little slower than normal but ok. We thought maybe she just ate something that was hard to get through her system. Saturday morning though she became lethargic and wouldn't eat, so we took her into the emergency vet. They took xrays (link below) that showed something pervasive in her lungs. She was admitted to the hospital, put into an oxygen chamber, and given a broad spectrum antibiotic, but apparently it wasnt enough because she stopped breathing later that night while at the hospital. The doctor said it could have been previously undetected cancer, pneumonia, blastomycosis, or possibly (but unlikely) even covid (there was covid in the household around this time). Unfortunately she died before anything could be determined. I can't get over the guilt of not taking her into the vet sooner and wondering if that would have helped. What's more, before this happened, she seemed so healthy and youthful. She also never had a cough or fever that week, though she did throw up a little liquid that Tuesday and seemed to feel better afterward. If any vets or specialists could offer insight as to what caused my little girl's death, I would be so grateful for the peace of mind.

Hello!  - and I'm sorry to hear about the loss if your girl.  I'm afraid that second opinions, especially specialist opinions, do have to be obtained through a vet with the medical facts and paperwork, having been authorised to do so by your own vet.  I therefore haven't opened the picture that you have so carefully provided at this stage.  If you need a diagnosis, it may still be possible to arrange referral of the radiographs or even, depending on timescales, a post mortem examination.  However, this may not be what you most need - and you have the luxury of choice. You write as though you would like the referral vet to say: 'This pet wouldn't have lived even if she had presented earlier,' but of course some respiratory diseases are treatable - or perhaps, manageable for a while - especially in the early stages.  Some illnesses do advance quickly and lead to death in an otherwise fit individual.  Others (diaphragmatic hernia, for example) can come and go quite abruptly.  Respiratory distress is often an emergency. But is it your fault?  This is a different question.  If you'd have recognised this disease as something that might have cost your girl her life in this way, you would have been at your vets the moment you noticed it.  Would most reasonable non veterinary trained owners, faced with the information / understanding you had at the time, have headed straight to the vets?  Possibly not. How much would the visit have changed the long term outcome?  Possibly not at all - most dyspnoea patients have a very short survival time after first presentation. It is absolutely normal to feel like this after a sudden death.   Indeed, looking for somewhere to attribute blame is one of the stages of grieving.  Kind people who have faith in others often choose to look for reasons to blame themselves.  Sometimes, whatever the radiograph says, they will still find a way to blame themselves. Rather than a second opinion, I wonder whether what might help you is to talk through the case and your grief with an empathetic veterinary professional or counsellor who may even be able to read the history.  The Ralph Site, a website charity started by the lovely Shailen Jasseni, might be a good place to start.