Knee creaking, stiffness and pain..but it started kind of suddenly?

I did not hurt my knee nor have I ever had knee symptoms but say 1 week ago it started creaking/crackling when I would walk up stairs and then several days later it is stiff like the joint is trying to lock up on me and when I flex my knee around it gets painful, if a doctor is reading this then great but I ume not so I was wondering if anyone has ever experienced this and what did it turn out to be? I'm not that old.. only 33