To echo what's been said so far:
It is okay to say to your mother,
- "I don't know why, it just is."
- "I don't feel like I have any choice about this, it is just who I am."
You may also want to share the fact that you were worried about her reaction and call her on it if she is acting "mad". After all, she said that she wanted to be supportive, right?
So, if she gets mad, try to stay calm yourself (often fights can escalate, even though neither party wants to be fighting, simply because people are hurt by someone's reaction). If you are calm, then you can ask, "do you mean to sound mad right now?" Or say something along the lines of, "Mom, I really need your support because I'm still young and just starting the process of becoming an adult, but, right now I am scared because you sound so angry." Etc.
And, to echo again, PFLAG could be good for both of you. Give them a call.
I might also add that the "why" doesn't really matter.
My eyebrows are different colours and I get asked why all the time. I simply say, "I don't know, I was just born with it." There have been times in my life when I have explored the why of the genetics of it, but, nothing changes the fact that they are different.
If you can, try to think of the coming conversation with your mother as an opportunity to share and get support (you can even tell her that this is your goal at the beginning of the conversation) rather than a confrontation that you have to have.
If you are looking for other adults who might be able to be supportive, consider teachers. I teach high school and I have had many students come-out to me. For several of them I was the first person they told, but after opening up to me they were able to discuss their feelings more widely. Teachers, also, may have access to information about where you can go for better support.