Why dont i gain any weight? why dont i grow at all anymore?

Junk food will definitely stunt your growth. Also, the fats in these foods store fat away and it will eventually catch up to you and you will end up like your mother. If you start eating healthy, lots of fruit and green vegetables you will see a significant difference and your body will respond positively to it. At 11 and 12 years old age, it is CRITICAL to do good for your body because it only goes downhill from then on.