My fiancé's mother interfered with my plate

I've been on a diet for a week, and my fiancé took me to meet his mother. It was a lovely environment, but while everyone was preparing their plates, the woman who will be my mother-in-law took my plate and said, "That's enough, don't take too much, let's not get fat."

I couldn't say anything in front of others, but I was really upset. Should I have gotten up and left at that moment? What should I have done?