I am so sorry that this happened to your precious little one.
There are other members here that have also lost their beloved babies in this way. My vet will not spay or neuter until 6 months and a minimum of 5 lbs which is what Doodlebug was when he was neutered. Bogey was 5 lbs and almost 2 years. Indianagirl lost her Jezebel last year following her spay and also feels the way you do. Such a sad and horrible thing to have happened.