How do you discipline?

My mom is very easy-going. I have a ton of freedom. Anyway, my boyfriend walked me home from school like always, then he went home. When my mom saw me come in, she said go wait in your room. I was in my room doing my homework for about 10 minutes and she walks in. She told me that I was in trouble. When I asked her why she looked very mad. She pulled down my pants and hit my . Then she stood there screaming about how stressed she is, blah, blah, blah. It was super embarring because I was on the bed with my backside showing to her. How should I ask her what I did? Do you have an embarring story to cheer me up?