Was this right? Am I overthinking it?

My dog died, please help give me advice!! Am I overthinking this??? English is not my native language so please sorry for any mistakes in my text. Okay so my dog went to the veterinary last week: June, 8. She went there because she started having shortness of breath 2 days before, and we couldn't take her earlier because it started satuday night and then it was sunday. She was breathing as if she ran a marathon. The vet told us that she had last stage cancer and it had spread to her whole body, there is nothing we can do. The vet told us that she wouldn't get past the end of the year. The vet told us she had an infection and gave us 3 syringe with white liquid to apply on the soft skin at the back of her neck, just like a vaccine. And this is where I'm trying to get at... Day 1- we applied the injection and she screamed with pain, which we knew would happen, ok normal, medicine applied successfully. Day 2 - we skip, we had to give the injection every other day. Day 3 - we apply again, this time she didn't scream, just a small discomfort. Which was weird. Day 4 - her symptoms only getting worse, and worse much much faster. No inection this day. Day 5 - we apply again, she didn't scream at all, actually it was like she didn't even felt the injection. Her symptoms only getting worse. Day 6 - she still getting worse and now her breathing is faster like she ran some marathons, stopped eating and drinking water. Day 7 - in the morning she died of "natural causes", probably a heart attack because of her fast breathing rate. Exactly one week after visiting the Vet. Is this normal? Am I overthinking it about this medicine we had to give her? Sorry but her health deteriorated so fast it's weird, we know she had last stage cancer but she was "ok" until her breathing rate increased. But it wasn't that fast, it was just a faster and deeper breathing, but in one week it went to a complete racing breath and then she died.

I am so sorry for your loss and wanted to reassure you that "overthinking" and replaying it, and feeling either blame or guilt or anger (or all at once), is entirely normal in the wake of any death.  I like to think of grief as a thicket of conflicting feelings and confusion.   You are doing exactly the right thing in trying to understand what happened and find answers to your own - very valid, given your observances - questions. It sounds to me as though your vet believes that your loved one died because of fluid or masses on the chest.  A progression from panting to moderately heavy breathing, getting slowly worse, could be consistent with cancer - or heart failure or fungal infection or one or two other things besides.   I don't know from what you've told me how your vet knew it was cancer in this case - perhaps they'd done tests, or felt a lump on the examination, or taken pictures, or heard changes on listening to the chest that indicated that.  You'd have to ask them. Nor do I understand from the description that I have what, if anything, was done to stabilise your dogs' breathing?  When animals present in a dyspnoeic (struggling to breathe) state, Oxygen therapy (and sometimes a chest drain or lasix to remove fluid from the chest) might sometimes be used to try to stabilise things and make them more comfortable, particularly if they appear to be deteriorating.  Antibiotics treat infection; in some situations where there is an infection, antibiotics may help to prolong life.  It depends why a dog can't breathe, what can be done to help relieve the breathing.  These are all palliative treatments that sometimes improve symptoms. Primary treatments for a cancer include things like chemotherapy, which is frequently palliative also (causes relief rather than makes things better).  The success of these depends on the type of cancer and how early it is caught.  They are sometimes used  in veterinary medicine. I'm so sorry about the loss of your dog and hope that this has helped a bit; however there is no substitute for going back to your vet and asking questions to understand what happened;  why they felt that your pet deteriorated.  It may help to offer some closure. Liz, BVSc but not MRCVS