My hip hurts and i have no idea why does anyone have ideas?

so the other day i was walking to school and all of a sudden my left hip joint/ started hurting. it wasnt so bad but the more i walked the more it hurt. im only 15 so i dont think it could be like arthritis or something like that. it started like 2 or 3 days ago. it only hurts when im walkin or moving that hip. any ideas on what could be wrong?? i havent hurt myself or nothing so its a mystery to me??