Pulled leg muscle??????? HELP PLEASE!!?

I dont know when it happened and there was no sudden pain but my left leg has been hurting. most of the pain is about on the left side of my leg and 4-5 inches up from my ankle. it starting bothering me a week ago and the pain is getting worse. i am thirteen, very very active, and i havent told my parents becuz theyll just say itll be gone in a few days, but i dont think its getting better, maybe worse. the worse pain is when running. after doing a lot of activity on it it hurts for a half an hour then when i rest it the pain is only a 1 or 2 out of ten. when i run its a 8 or 9. walking is 7. any ideas on wut it is. much appreciated!! Also depending on the injury u rate it wut is the treatment. could a pulled muscle or torn ligament need a boot and crutches. please help thanks