Had BC, but genes tested negative

I had BC and got a double mastectomy. There is no history of BC in my family, and my gene testing came back negative. I was only 31 when I was diagnosed, but looking back on it, I know I’ve had BC for several years before that. Does anyone know how I could have gotten BC so young with no family history or gene mutations?

There are probably lots of genes that can cause breast cancer that they don’t know about yet. Also, when they tested you, which genes did they test for?