Cant move leg? (already asked, just more info)?

Okay so im 14, 5'4 n 110 pounds. When i bend my knees completely/sit on the ground, my leg will literally lock. Example: i was camping with my friends and there werent enough chairs so i knealed, when i tried to get up my leg just locked, it was extremely painful. i had to have my friend yank my leg straight because i was scared...this has happened a couple more times.. Its like, a pole-like feeling. I can feel it in my leg when it locks.. whats happening?