I agree with Rraa, when kittens go to a new home this is stressful and can make them quite upset
it is a hell of a lot for the poor little soul to cope with,
also Rraa is quite right about each home has there own bugs this is just another thing for the kitten to cope with
Some kittens feel this worse than others.
Most breeders sell kittens with 6 week insurance starting from the day they leave the breeders home.
I think the breeder is being very responsible offering to take the kitten back,
this is more than some do
As the breeder is doing the right thing by you and the kitten you should keep this between you and the breeder just mho
I do hope your kitten will soon be 100%!!