Suggestion about cancelling my visa

Hello, I applied for my child visa who is under the age of 14 years old for the interview waiver program. after applying they called for an interview then I went for an interview they asked only one question how long have you been staying on your last trip to New York I answered that 3 months he didn't ask any further questions if he asked then i could explain why i stayed there for three months then he cancelled without prejudice stamp on my valid visa