I am 15 years old, between five and five-one feet tall, and weigh 120 pounds. Am I fat?

I run and lift weights so I know a lot of it's muscle, expecially in my upper body, but my stomach is too big. I wear a size 5 in jeans! I do 100 sit-ups every other day, but it doesn't seem to help. How do I get a flatter stomach? Everyone says I'm "just right" and that I'm little, but I know it's not true. Am I fat? Please help!