Checking in with you here. I'm extremely sorry to hear of your difficult situation. There is a lot going on. I'll try and sort it out a bit and hope that helps. Just my thoughts--so see what appeals to you--this isn't gospel!
Breast cancer diagnosis can be a shock, even more so in your circumstances. Fortunately this is a very early diagnosis. I would directly ask the oncologist if you can have some weeks before treatment. It will take a while to set up in any case. Also, get as much info as you can about what would counter indicate breast feeding. I also think a lactation specialist might help. Also, if you do need to wean, ask the pediatrician as to how to do it in a way that isn't too jarring to you or the baby.
If you are at a cancer center, a social worker in oncology might help you sort through the issues. As a new mom, you are vulnerable, with hormones still adjusting. Do you feel the oncologist fully understands your personal situation? Can you OB/GYN offer any advice?
I can relate as I was told to wean my daughter at 3 days because I needed gall bladder surgery. I was able to work it out, but only because my PCP was supportive of the breast feeding.
I certainly understand your fears of treatment, but here again you don't have to rush. Meet with the oncologist, find out what the suggested treatment plan is, and then evaluate. Do you have a partner, friend, or family member you can depend on? Try to bring someone to all medical appointments, and have them take notes.
I hope you'll get some good advice here. Do check in when you feel like it. Will be thinking of you.