I'm so sorry for the loss of your fur-baby.
And don't feel responsible. When we lost our dog, she had been feeling poorly and my Dad had taken her to the vet. During the middle of the blood draw, she did the same thing as you described. The vet believed that she had a heart attack. When their little bodies are under stress lots of things can happen. But what gives me comfort is she died in the arms of someone who loved her. Just like your Diana.