insisting that she doesn't know me anymore
To take the edge off it, among other things, you may want to assure her that your values have not changed, so she does know you almost as well as before. You still love her and the rest of the family, you still grieve for the person who passed away, you do not steal or lie or "sin" in whatever other ways, at least not any more than before your deconversion. Stay friendly and positive, ignore any hurtful remarks you will get (if you have trouble with it, think about it from her point of view: her world has just been turned upside down, no wonder she has trouble staying calm).
Once she is ready to talk, you can find out whether your faith is more important to her than, say, your integrity (discussing an example of a devout Catholic who committed a transgression can be useful there). Encourage her to discuss the issue with her pastor, maybe. Under no circumstances you should be sucked into discussing the merits of Christianity, not at this stage.
It happens on occasion that a person considers atheism (or homosexuality) a sin worse than murder, in which case you may end up severing your relationship for a time.
Yesterday my mom noticed (at a funeral) that I wasn't praying or participating in the mass. She confronted me about it, and I told her that no, I am not Catholic. Apparently it's sinking in and she's a bit hysterical... crying and screaming that she doesn't know me anymore.
What do I do? I don't know how to react/behave when she's doing this. It's like she wants me to feel like I'm doing something wrong, but it isn't working, so she's getting hysterical.
*edit*
I gave her a hug when she calmed down and told her I love her. That seemed to help, a little. Based on her previous behavior in situations where I've done something "wrong," she will (in the future) make barbs and slight passes at my beliefs. (Already she made one: insisting my love of science is causing my social anxiety disorder.) The advice given in the comments is really helpful. I plan on making the most of it.