What do you say when someone asks you where did you get that fat ?

No lie .My boyfreind wanted to meet with some freinds of his and begged me to come along .Well his one freinds wife was sitting next to me and I smiled at her and her husband at the same time. I thought we were cool.My boyfreind was on my other side watching the band when she says real mean so where did you get that fat ***? She is really skinny and yes Im a white woman with ghetto .Ive always been that way. I just wanted to cry.I felt slapped because I had put time and effort into looking nice and yes I outweigh her by 50 pounds Im also 6 inches taller.She is freinds with my boyfreind and acts all buddy buddy and Im just disgusted.What should I have said? I said nothing.I turned red I felt like I didnt belong and I just asked my boyfreind what he thought of the band. I should of said something!