Why am i angry at this friend (not to her face)?

This doesn't sound like the kind of person you'd like to hang out with... so stop hanging out with her. Your reasons for anger are valid, just give yourself some space and avoid her. If she does something that annoys you just calmly explain what she did, why it bothers you and go on with life. If she gets offended, that's her problem, if she changes, good for you! As for why all of these emotions are coming out all of a sudden... well you've probably been feeling them moderately, but now everything is coming out full force. Irks and irritations add up into a big fury... which would be what you're feeling right now. You'll get over it pretty quick, but as soon as you're out of her company the ire will return. I had a friend like that, and we hung out for a long time, and she beat me down. Now I just don't hang out as often, if at all and I have better friends. Life works out better that way.