if I had to guess...... breathing issues like acute asthma, it comes and goes real fast, nearly lost my child many years back. Not something you may notice as your child may be crying a bunch and then work up the asthma to go to sleep quietly. Because you think they're just resting, you let it be, but they may be sleeping because a depletion of oxygen (makes them tired real fast), and it happens within hours.....
it was my third child no less, and I didnt immediately spot it, but did and went to the emergency room and let the facility know breathing issues, they'll get the child in immediately, had a team on them right away, oxygen mask, IV, the works, all on a tiny little baby.