Today after lunch my mother confronted me in my room. Well, she didn't so much "confront" me as stand in the doorway asking me questions.
I'll have to explain the relationship between me and my mother for this to make sense. She's from South America, and English is not her first language. She's also Catholic, and very much into her religion. When I was about 11 she became aware that something was "not right" about my sexuality, and was not pleased about it. She thinks that anything other than straight marriage in a traditional family and such and that, is a choice people deliberately make because they're mentally ill.
I think this is ironic because she has a particularly severe case of bi-polar disorder, been in and out the mental institution several times because she gets dangerous. I have to take care of her a lot of the time, drive her to the doctor at least once a week (usually more) to see a variety of doctors. She's very sick with a lot of things.

So she's standing in the doorway of my room, and asks me, "Why do you cut your hair like a man/why do you dress that way/etc."
I let her finish her barrage of questions, and simply said, "Because I feel like it."
Nothing else. I'm not going into detail about it to have yet another argument, I don't have that kind of patience or energy.
She looked at me for a long time and left the room.
It's a tough situation because I'm grown but still living at home. There's no way that feels legitimate to transition while I'm still living here. I feel guilty because in a sense I did "choose" to be this way, at least by taking action to look more male. It's stupid because I've been dressing the same way for a very long time now, but just haven't been cutting my hair (not AS short anyway, within the range of androgynous since forever...) or hiding my boobs till this last year or so.

Um, I guess advice is totally welcome, although I'm venting more than soliciting for actual help.
I really don't want to come out to my mother because living with her is already very stressful all the time because of her mental illness and lack of ability to take care of herself.

For what it's worth, I'm not to keen on coming out to my mother either... the 'I feel like it' excuse is gonna have to cut it for me, too.

move out.

@ Mister:
Oh, I plan to. It's tough just getting a job interview though. Nearly every business is on a hiring freeze, and I'm not in school (and blew my grant by not getting good enough grades the first two go-arounds, so that's not an option at this time either). I'm pretty stuck here for the time being.
But given the right circumstances, income, etc, I so would if I could, in a heartbeat.

@ CodyJess:
Thanks. It is somewhat reassuring to know that others do go through this.

If she's as unstable as you describe, it may be best not to come out to her until you are 100% self-sufficient.  Sorry, but 'get a job' is really the best advice I can offer, and I know that getting a job is tough for just about everybody right now...