I feel like a terrible mom 😢😢😢💔

So ever since I got pregnant I noticed changes in my 6 year old daughters behavior. She's not jealous of the our baby's arrival on the contrary She's thrilled. But she's just been acting up lately. Slamming doors, talking back to me, when we asked her to clean up her room she doesn't, when it's time for bath she throws a fit. She has never acted this way and I never really had to spank her. Usually we talk to her or take away her favorite toys, loose tv privilege etc. Today, I just lost my patience. I was baby sitting my two year old sister (she's a surprise because I'm 22 years older than her) and my daughter purposely threw her on the concrete floor playing outside and now my sister has bump on her forehead. So anyway, I took my daughter inside gave her a big spanking on the bottom and when I hit her the second time she moved and I hit her arm. And now I'm balling my eyes out because I left her a big hand mark on her arm. And tomorrow She starts school 😢 how do I make the swelling go down? Will it be a bruise? Am I a horrible mom for spanking my child? I feel so heartbroken right now, the mark has gotten darker and more noticeable   💔