Was my boss at work overreacting by giving me a warning for writing personal notes?

I was at work and I am usually never slacking but there was a song stuck in my head I wrote down notes on and all of a sudden my boss comes over to my desk and looks at me weird asking me what I am writing or asked me why I am not working. She saw what I was writing and took the paper away and threw it out. Not only did I get in trouble for writing personal notes not work related but I also forgot the name of the song that is stuck in my head thanks to her throwing it out. Was she out of line here? I mean people talk and slack all the time and they never get in trouble. I do one thing and get told about it.