I have an injury, and it does not seem to heal. Any advice?

Hi, i'm a 13 yr old sprinter. I started running this year only...at January. The previous week i ran, which was on a sunday, i think i strained or pulled a muscle. I dont know what injury is that, but it is probably in my hamstring group. The problem is that when i run and push myself forward using my feet, theres a sharp pain that can be felt under my gluts. This stops me from running. What injury is this? Can you guys give me some advice? Like how long it takes to heal...how to strech/prevent this injury... what do i do to make it heal faster. I posted this because the injury doesnt seem to heal. For 2 whole WEEKS! I really want to get back to training. I want to win next years sports meet. What should i do? The injury doesnt heal!! Or is it because i have been running with the injury?? Everyday, i take small strides or even sprint to see if the injury healed. After the exercise, i can feel the pain again. Should i exercise lightly, or do nothing and rest? Thanks...