Mean friend what should i have done?

we went out to eat and my so called friend swapped burgers while i went to get her a straw because her burger was hard and had less sauce(i know this is stupid but still i believe in fairness) .my other friend she said hey mystery while you went louise swapped ur burger because i told her and they all started laughing at me.im really shy so i didnt say anything but i dont like peole touching my food.so i didnt say anything.i am sensitive and it hurt me when they laughed plus it was my first time eating out because i never eat out.what could i have done?what would you have done?if i said to hey thats not fair she would be like oh my god why are you crying over food?plus she thinks shes so big she pushes others around and treats me as lower cl.