My knee hurts really bad?

yesterday i went shopping and we walked around and stuff but it didnt hurt at all during that period of time, we took bart home and as soon as i got off my leg hurt a little bit but i didnt know what from, as hours ped it got worse and it was hard for me to climb stairs. i thought sleep would help but i woke up and the pain is even WORSE and i cant climb stairs without collapsing, standing up off of bed, chair etc. is just painful, walking hurts too, and when i straighten my leg or lean on it, then stand back up its just terrible pain, can you tell me whats going on here? im 14 if that helps at all?