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I agrEe thanks for the advice yes I know about all the screenings. I spoke to the breeder and she said that the kitten was vet checked twice before coming to me and was healthy also that maybe it was from stress of moving and then going to the vets. She asked me to go again in 3 weeks or get a second opinion and then get back to her. Can this be the case?

She is from milwood Calcutta koppiekatz spotsalot gogees starwinds lines

A vet check of a murmur is only as good as the vet doing the examination. Some vets hear murmurs easily, others seem to struggle. It is all about experience and I presume their hearing acuity. A vet check is no guarantee on its own of cardiac health.
Cats with murmurs can be well cardiologically and cats with no murmur can have serious heart disease.
Hence the need for scanning and a cardiology opinion. It is the only accurate cardiological assessment of a cat on the day. It is no guarantee of the future but at least it gives you a start off point.

Stress can cause murmurs, but stress can also show up murmurs in cats with cardiac trouble.
A three week re-assessment is not going to do any harm, but it may not give you the answers you need in a cat that is going to be used for breeding.
If you decide to keep this kitten, I would make sure that this breeder will stand by you when you take the kitten for its pre-breeding scan, as it would be dreadful to wait until then and the breeder reneges on the deal when the kitten scans positive for heart issues at ~one year of age. I would get any pledges re refund/compensation/replacement in writing and signed just like a contract to take into account the fact that the kitten has a murmur now. You do not want to waive any rights to compensation, if you decide just now to keep the kitten until it gets its pre breeding scan.

Thank you so much for the advice, I think I will keep her for now if the breeder agrees that if she has any heart problems in 6 months time then she will refund me or replace her. It's a shame as she is a very stunning cat. Once again thanks so much

Some kittens have signs of HCM at young ages, the youngest I have heard of was 4 months old.
Scanning will identify those kittens with other heart problems of course.

Some kittens/cats with HCM will not have a murmur at all.

Unless a kitten has a very loud murmur or symptoms, then I feel the scan does not need to be done on a very young kitten, as some of those murmurs will be "innocent". However I do not think you can assume that a cat whose murmur disappears, is a suitable cat for breeding without it being seen and assessed by a cardiologist before breeding.

I do feel that ALL cats should have a scan before breeding as that will pick up those with HCM and those with other types of heart disease which can also be genetic and capable of being passed on to offspring.