Why did the doctor looked like she didn’t believe me when I said I’m not sexually active?

I mean when I went to the doctor two days ago and I had to leave work early, she was asking if I am sexually active and I said no. She looked at me and asked if I’m sure and I flat out said yes. She said she asked that because most people around my age are but she said that’s good that I’m being careful. I’m going to be 20 next month. I mean I’ve been in relationships in the past but we’ve never done anything sexually. I don’t want to get pregnant at a young age and a guy leaving me. Screw that.