Am so sorry about your mum (my mum died when I was in my teens, so really feel for you). However, her dying of leukaemia isn't really relevant in this instance. The reason I asked was because if it would have been ovarian cancer, or even pancreatic, then there might be a possibility you have BRCA 1 or 2. Even so, when you fill in the form re. your medical history, state everything - your grandmother and aunt's breast cancer is relevant, and they might well still look at your genes to see if you have any other mutations - I recently discovered that there are some 92-200 gene mutations that can cause breast cancer. But at the same, most don't have the mutation, and it's just a matter of something else entirely.
Please, try to stay away from Google or even the internet. There's no point reading up on something that might cause more worry. I say this from a place of love - I have breast cancer - I was also on HRT*. Have barely googled anything, partly as I would rather keep my mind free from cancer as much as possible, but want to get all my information not only first hand from my medical team, but based on ME. We are all unique, and when it comes to cancer, yes, we can get an idea about the critical mass, but ultimately, everything is absolutely about you as an individual. So there's no point reading up on anything until the doctors give you actual solid facts based on actual tests on your own tissue.
Lastly, yes, the odds are in your favour to the degree that is more than likely nothing. While it is perfectly normal to panic and assume cancer if you have a breast lump, it probably isn't. So please, work on your anxiety, try to quiet your mind as there's really now point worrying while things are still up in the air and you don't know the facts pertaining to you. Also, and perhaps most pertinently, breast cancer is now hugely treatable (and survivable), even more so if it's caught early. You are have an appointment with the breast clinic and they will investigate. This is the most important thing in all this.
*I took HRT in the knowledge that some cancers are fed by hormones. Given my family's medical history, and my own, I went to see an HRT specialist and took the lowest dose possible. I don't regret taking HRT, and would absolutely do so again. If anything, am more upset about having to stop taking HRT (which was my choice btw, my medical team did not tell me I had to) than I am about having breast cancer. I don't blame myself, so please, don't blame yourself for taking HRT as its benefits can really outweigh any risk, and to me, that's gamble worth taking.