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.