I called a random girl a fat , is this bullying?

I was with about 6 other boys at school walking down the corridor, I was at the front because I'm the leader of our crew. Anyway, we were walking and this loner girl walked past us, as she did I got up to her ear and shouted "FAT !!!" at the top of my voice. Everyone laughed and she cried and ran off, even the teachers were laughing, I considered myself a hero that day, this was 2 days ago. Was I bullying