I have this really bad pain in my lower back, the left side.?

I'm only 15 years old, and I'm very active. I am involved in sports and have never had a problem. Today I was working outside with my horses, then when I was coming inside my lower back started hurting really bad. It kind of hurts to walk and I can only sit one way, straight up like my back is against a wall. I took some ibuprofen and have an ice pack, It's helping some, But I would like to know what is wrong with me! Please help!