Also: have you checked for rabbit mites (a skin scrape)? I know of several instances i which puppies that were scratching frantically actually had these, not fleas. They often congregate towards the tail. One of my puppies came from the breeder with them. Vets can be surprisingly unfamiliar with them. Ear mites are the other thing to check for.
As with fleas, the environment must also be treated -- bedding put through a boil wash etc.
Given the screening done by your breeder I agree with Flo and Sins that it would be very unlikely that you are seeing SM at such a young age. (But as you also recognise, it always remains a possibility over the lifetime of a cavalier. Making a careful choice of breeder as you have done is the best possible, responsible approach. )
I am sure this will be a matter of environment/pests and not SM.