Why is my mom angry all the time?

Lately, my mom has been angry, irritant, and annoying. Everyday i'm in my room being a normal teen on my bed using my laptop and watching tv. She calls my name from down the stairs to do something and before I have time to get off of my bed she calls me again. I find it VERY annoying. Then, every time I try to talk about something she changes the subject and then yells at me for something that I was supposed to do about a week ago but never had the time to do. For the past few days, I've been worrying about my hair so since my mom is the only one in my house who knows how to do my hair and can drive me to get hair products (I got my dad but when it comes to hair he knows NOTHING). So, I've been talking about my hair a lot because I'm looking for hair products. She told me that she had coupons for John Frieda Frizz-Ease and so I've been looking at their products. Today, I went to talk to her about one of the products and she starts screaming at me and telling me to leave and all that. It made me cry because I don't like being yelled at because I'm a good kid at school and I get all A's and I DON'T think I should be treated like this. It happens all the time now. With everything. Not only to me, it happens to my brother too. It's not fair because she has been talking about something for more than a month to me so since I'm the daughter I can't really do anything but listen. Sometimes I just feel like running away but luckily my dad is there to help me when I feel down about my mom.