HUGS
I am so very sorry.
If diaphragmic breathing is the first symptom you notice, you are generally very much up against either one of the nastier respiratory bugs or against acute heart failure, and even with medication the outcome would have been dicey.
I have been sitting here keeping my fingers very firmly crossed in the hope that Miga could hang in there long enough for you to see a vet and for any medication to kick in. So very sorry that the timing was the worst possible.
Please feel sad and upset, but try not feel guilty or like you have failed Miga. You have done your very best under the circumstances, and that is all you can do!