Can anyone tell me why my knee is hurting? (10 points)?!

Okay so for the last week ive been having this pain in my right knee when i stretch my leg. Whether im laying in bed or sitting in a chair if i try to stretch my leg out i get a throbbing burning pain in my knee. What really makes it hurt is when im walking up the stairs i literally cannot put any weight on my right leg without my knee feeling like its going to break or something. The funny thing is as long as i keep my leg bent its fine. can anyone tell me what this might be i have no idea where to go from here? Thanks in advance.