Can someone help me with this diagnosis?

So my knee has been hurting pretty badly lately. The shin on that leg is swollen and it hurts to bend the knee. after prolonged sitting it hurts alot to move but if i lay down for a while and then move around it feels fine. it gets worse in the cold and has been steadily getting worse with time. can you guys help me out? so extra info- I am in the army rotc and do lots of running...I know it seems obvious but I have been running for years and never had something like this.