Why does my right leg hurt so much?

Hello, I am 13 years old, and within the past week, my right leg has hurt SO much. It feels like my muscles are being torn, I can't really describe it, it just REALLY hurts. In gym today, I thought I was gonna pass out, but there was nothing I could do about it. I don't play any sports, I'm one of the skinniest boys in my class... I don't know what it could be. Anyone have any suggestions?
EDIT: It's not growing pains, way way different.
Asked Sep 23, 2015
Edited Sep 23, 2015

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