I just don't understand

Tonight I broke the news to my mom that I wanted to talk to my doctor about getting on hormones. She seemed pretty upset stating "No." and that if I wanted to do something like that, I have to do it on my own and she doesn't think our insurance will cover it anyway. I'm fine with that because I have a job and am old enough to do things on my own anyway. I know these kinds of things are hard on family no matter what. I think she thought i'd just bind my chest, keep my hair short, choose a "nickname" for myself and that'd be it. That's what Transgender is.
No.
And I think tonight my announcement that I wanted to go on hormones jolted her. We didn't get to talk about it because she ran off to bed after just those things said. I just don't understand how this is so hard for her. I'm still trying to wrap my head around how she still sees me as a girl when I NEVER acted girly in my life. I just don't understand how she sees me as going from a girl to a boy like BOOM. I've always dressed in my brother's clothes, I've kept my hair boy-short since graduating high school, I told her I was Trans 7 months ago. I just don't see how this could be so shocking for her. She's never had a daughter. She's always had another son, just this one with "defective parts". I mean, yeah, I wore dresses for dances and stuff but she always had to wrestle me into them. I just want to understand how she sees things and why but it's pretty hard when whenever I throw something at her she runs away.