Welcome to healthcare Magic!
I am really feeling sad to know that you are bereaved of you boyfriend.
The deep sigh like irregular breath with nodding are called agonal pattern of breath and is indicative of a very critical clinical status.
such breathing pattern is common in severe metabolic disorders, drug overdose
, hepatic failure
, shock etc.
is a slow disease and per se would not cause such acute death.
I hope the advise would be informative and useful for you.