My knee hurts when I dont move?

Ok I'm currently 12 and it started way befor I could read cuz I remember wakeing up one night my knee swollen and in deep pain. It's been going like that for years but any way I play softball and soccer and I'm a klutz but I used to be able to handle it by moving my leg or massaging it but lately it's been getting worse u don't know what to do!