Have a week old baby, can't cope with my 2 year old

I have a 2 year old little girl that has always been very good. Until her sister arrived a week ago. She has completely changed. She's naughty all the time. I know this can be normal but I can't cope with it. I cry all the time because I'm so upset about it. I'm sure she hates me! She has started biting and hitting me. She constantly just tells me no and whines and cries all the time! I feel like a terrible mother because I smacked her earlier! She was pulling my hair and I couldn't get her off so I pushed and smacked her on the arm, I'm a complete failure and really struggling. I can't stop crying 😭