My mum says I have to gain weight, but I don't want to?

My bmi says that I'm underweight for my height, but I'm not really that skinny, but I'm happy with my weight. My mum has been taking me backwards and forwards over the past couple of months about my weight and they referred me to a child pediatrition. I know she's only worried and wants what's best for my health, but everytime I tell her I'm happy with my body, she tells me I have to gain weight. How do I get through to her?