What should I do about my knee?

I am 17 years old, over weight and semi active. Im in the marching band in the school. Early May in physical education class my knee buckled out from under me, without warning. I was standing still with my knees bent, the knee popped out and I fell, couldnt bear weight on the knee. This was the first time. I was on crutches for a week. The Second time I was marching in band, changing the direction from foward to backwards, my knee popped and I went down, intense pain was felt on the inside of my left knee, the same knee that gave out in may. I was taken off the field in an ambulance, I go to the er they did no xrays or anything but tell me to see my pcp. I go see my pcp and he tests the mobility declaring I sprained my MCL ligament and it will take a few weeks to heal. This past weekend I was preparing for a holiday party for my community and my knee gave way when I want to take a step backwards. It popped again and I went down. However the only difference is I didnt need crutches as the past two time, it was just tender and sore. Can anyone give me a suggestion as to what to do? I really dont want to go back to the PCP.