Why did she act like this?

I had this teacher that always seemed nice and everything. She let me go anywhere and all. She just seemed like a motherly figure I guess. But then I got in a little trouble w/ her and she started acting all tough towards me. She’d be a smart a$$ and once she pushed my head down for no reason. She would also stomp her foot at me, but then smile about it. It seemed that she liked torturing me for the fun of it. She also teased me, and said she would tease me. Sometimes I’d ask to go somewhere, and she’d say “no” and then smile, as if she was scared to get mad at me or something. How could she go from being nice and caring to being a monster over night? She treated me like that for the rest of the school year. Don’t say I should tell my parents, I just want to know what was up with her. I don’t have her next year so I’m not going to worry about telling an adult, I’m just curious as to why she’d change her behavior. I didn’t do anything bad when i got in trouble either