My Mom is acting Psycho! Lately my mom's been getting meaner and meaner....?

My mom accused me of losing her jean jacket, even though I gave it to her like a week ago so obviously she lost it, and then I asked my brother to defend me, and he just sat there like an idiot. He's also really annoying because he's older than me, I'm the youngest child and I'm a girl yet HE's the really spoiled one (I say this because most people assume that just because I'm a girl and the youngest I must be spoiled). I always get butt whoopins and it's only because my mom is 24/7 PMSing. Once I wasn't good at playing tennis and she thought I was playing bad on purpose, so she grabs me by the front of my shirt like those bullies on TV then shoves me, slaps me really hard and just yells. I was 12 when that happened. And whenever my dad asks about like a mark on my face or something, or a fight between us, she just says "Oh im soooo sorry, I'm on my period blablabla" and then everyone else just forgives her for turning into a demon over a little period. Seriously, I have like constant pain from what might be SCA and you don't see me turning into a monster. I was in the car today with my dad and my older brother, and I was asking my mom if I could please have a little money for pants, and she goes crazy on me! She was like "You can buy your own pants youre 14 why dont you just move out!!!!" she was yelling and hissing like a rabid animal. I would understand having to buy my own pants if I already had a good amount of clothes and was just wasting money on the "latest trends" and such, but I only have one pair of jeans and a pair of short shorts and people make fun of me for wearing the same pants all the time. The worst thing is that no one else in my family defends me. Please help I feel like I was a mistake, if my mom wanted me, you'd think she would be nice enough to buy me more than 2 pairs of pants, then tell me to move out. It just hurts especially because I'm working so hard to save money up for college, and life after I move out. Am I in the wrong or is she?