Kitty might have been taking some "agonal breaths" - I hate that term, because it makes it sound like they are in agony, but they're not. But often after they have stopped breathing and have passed, they make suddenly appear to take a large gasp-like breath or large gulp - or more than one. People do it too when they pass. They have died, but the agonal breaths are just a normal reaction to what is happening. They are not aware of them, they are not purposeful, and they are not suffering. Or like someone else said - they don't stop breathing the instant the solution hits their blood stream - it has to reach their brain and sometimes, especially with very sick animals, their circulation is sluggish, so it may appear to take a while.
Please do not second-guess yourself. It is not right to allow animals to suffer if their quality of life is poor and there isn't really anything else we can do to make them comfortable. You did the right thing. I am sorry for your loss.