Family responses can be very odd. To say you are a glutton for punishment just sounds wrong. Nobody voluntarily takes on breast cancer. I suspect she didn't have any idea at all what to say and was awkwardly filling the gap. She is probably wondering why she said it too. I would take it as a 'blip' and see how she reacts next. Life has taught me that family is precious. I'd give her the benefit of the doubt. I think it stuns every mother when her daughter phones to say she has breast cancer.
I hope I never hear those words from one of my two daughters although I may have to. My husbands family carry the BRC1 gene and all the women get it, and now I have this. Their chances of bc are high. If I get the phone call, I will know not to say what you heard anyway.
Best wishes and I hope your mum supports you in your journey.
Take care.